SQL Server 2022

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CREATE TABLE Data (Id INT IDENTITY(1,1), DateText VARCHAR(100))
INSERT Data
VALUES
    ('3 Days 18 Hours 51 Minutes.'),
    ('3 Days 51 Minutes.'),
    ('18 Hours.'),
    ('1 day 1 hour 1 minute.'),
    ('10000 days 1 hour 1 minute.'),
    ('Undefined')

SELECT *
FROM Data D
CROSS APPLY (
    SELECT
        SUM(PAT.Multiplier * V.Value) AS Minutes
        --*
    FROM (
        VALUES
            -- The leading space is critical to the parse.
            -- Optional trailing 's' and trailing '.' are ignored
            -- Limits: 5 max digits for year, 2 max digits for hour and minute.
            ('% [0-9][0-9][0-9][0-9][0-9] Day%', 5, 1440),
            ('% [0-9][0-9][0-9][0-9] Day%', 4, 1440),
            ('% [0-9][0-9][0-9] Day%', 3, 1440),
            ('% [0-9][0-9] Day%', 2, 1440),
            ('% [0-9] Day%', 1, 1440),
            ('% [0-9][0-9] Hour%', 2, 60),
            ('% [0-9] Hour%', 1, 60),
            ('% [0-9][0-9] Minute%', 2, 1),
            ('% [0-9] Minute%', 1, 1)
    ) PAT(Pattern, Digits, Multiplier)
    CROSS APPLY (
        SELECT PATINDEX(PAT.Pattern, ' ' + D.DateText) AS Pos
    ) PI
    CROSS APPLY (
        SELECT CONVERT(INT, SUBSTRING(D.DateText, PI.POS, PAT.Digits)) AS Value
    ) V
    WHERE PI.Pos > 0 -- Match found
) C

Id	DateText	Minutes
1	3 Days 18 Hours 51 Minutes.	5451
2	3 Days 51 Minutes.	4371
3	18 Hours.	1080
4	1 day 1 hour 1 minute.	1501
5	10000 days 1 hour 1 minute.	14400061
6	Undefined	null