SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE [dbo].[SampleData](
    [ID] [int] NOT NULL,
    [SequenceID] [int] NOT NULL,
    [Sub Status Start Date] [datetime] NULL,
    [Sub Status End Date] [datetime] NULL
) ON [PRIMARY]

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 1, CAST(N'2019-08-06T00:00:00.000' AS DateTime), CAST(N'2019-08-06T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 2, CAST(N'2019-08-06T00:00:00.000' AS DateTime), CAST(N'2019-08-06T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 3, CAST(N'2019-08-06T00:00:00.000' AS DateTime), CAST(N'2019-08-06T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 4, CAST(N'2019-08-06T00:00:00.000' AS DateTime), CAST(N'2019-08-06T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 5, CAST(N'2019-08-06T00:00:00.000' AS DateTime), CAST(N'2019-08-13T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 6, CAST(N'2019-08-09T00:00:00.000' AS DateTime), CAST(N'2019-08-09T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 7, CAST(N'2019-08-13T00:00:00.000' AS DateTime), CAST(N'2019-10-03T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (288, 8, CAST(N'2019-10-03T00:00:00.000' AS DateTime), CAST(N'2019-10-03T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (404, 1, CAST(N'2019-11-02T00:00:00.000' AS DateTime), CAST(N'2019-11-03T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (404, 2, CAST(N'2019-11-28T00:00:00.000' AS DateTime), CAST(N'2019-11-28T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (404, 3, CAST(N'2019-11-28T00:00:00.000' AS DateTime), CAST(N'2019-11-28T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (404, 4, CAST(N'2019-11-28T00:00:00.000' AS DateTime), CAST(N'2019-11-28T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (404, 5, CAST(N'2019-11-28T00:00:00.000' AS DateTime), CAST(N'2019-11-29T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (404, 6, CAST(N'2019-11-29T00:00:00.000' AS DateTime), NULL)

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (239, 1, CAST(N'2019-06-25T00:00:00.000' AS DateTime), CAST(N'2019-06-25T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (239, 2, CAST(N'2019-06-25T00:00:00.000' AS DateTime), CAST(N'2019-06-26T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (239, 3, CAST(N'2019-06-26T00:00:00.000' AS DateTime), CAST(N'2019-06-28T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (239, 4, CAST(N'2019-06-28T00:00:00.000' AS DateTime), CAST(N'2019-08-04T00:00:00.000' AS DateTime))

INSERT [dbo].[SampleData] ([ID], [SequenceID], [Sub Status Start Date], [Sub Status End Date]) VALUES (239, 5, CAST(N'2019-07-01T00:00:00.000' AS DateTime), CAST(N'2019-08-04T00:00:00.000' AS DateTime))

SELECT ID,SequenceID,[Sub Status Start Date],[Sub Status End Date], DATEDIFF(d,[Sub Status Start Date],[Sub Status End Date]) AS [Duration Days],
    CASE WHEN Counts > 1 THEN CAST(1.0 / X.Counts AS DECIMAL(18,2)) 
    WHEN Counts = 1AND X.[Duration Days] > 0 THEN X.[Duration Days]
    ELSE 0 END [What The Result Should Be]
    FROM
    (
    	SELECT ID,SequenceID,[Sub Status Start Date],[Sub Status End Date], DATEDIFF(d,[Sub Status Start Date],[Sub Status End Date]) AS [Duration Days],
    	RowNum = ROW_NUMBER() OVER (PARTITION BY ID,[Sub Status Start Date],[Sub Status End Date] ORDER BY ID),
    	Counts = COUNT(*) OVER (PARTITION BY ID,[Sub Status Start Date],[Sub Status End Date] ORDER BY ID)
    	FROM SampleData
    )X ORDER BY ID,SequenceID

ID	SequenceID	Sub Status Start Date	Sub Status End Date	Duration Days	What The Result Should Be
239	1	25/06/2019 00:00:00	25/06/2019 00:00:00	0	0.00
239	2	25/06/2019 00:00:00	26/06/2019 00:00:00	1	1.00
239	3	26/06/2019 00:00:00	28/06/2019 00:00:00	2	2.00
239	4	28/06/2019 00:00:00	04/08/2019 00:00:00	37	37.00
239	5	01/07/2019 00:00:00	04/08/2019 00:00:00	34	34.00
288	1	06/08/2019 00:00:00	06/08/2019 00:00:00	0	0.25
288	2	06/08/2019 00:00:00	06/08/2019 00:00:00	0	0.25
288	3	06/08/2019 00:00:00	06/08/2019 00:00:00	0	0.25
288	4	06/08/2019 00:00:00	06/08/2019 00:00:00	0	0.25
288	5	06/08/2019 00:00:00	13/08/2019 00:00:00	7	7.00
288	6	09/08/2019 00:00:00	09/08/2019 00:00:00	0	0.00
288	7	13/08/2019 00:00:00	03/10/2019 00:00:00	51	51.00
288	8	03/10/2019 00:00:00	03/10/2019 00:00:00	0	0.00
404	1	02/11/2019 00:00:00	03/11/2019 00:00:00	1	1.00
404	2	28/11/2019 00:00:00	28/11/2019 00:00:00	0	0.33
404	3	28/11/2019 00:00:00	28/11/2019 00:00:00	0	0.33
404	4	28/11/2019 00:00:00	28/11/2019 00:00:00	0	0.33
404	5	28/11/2019 00:00:00	29/11/2019 00:00:00	1	1.00
404	6	29/11/2019 00:00:00	null	null	0.00