SQL Server 2017

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create table your_table (
 [date] date, 
 phase varchar(30),
 user_id int
)
insert into your_table 
([date], 	phase, 	user_id) values
  ('2020.1.1', 	'a',	10) 
, ('2020.1.2', 	'a', 	10) 
, ('2020.1.3', 	'b', 	10) 
, ('2020.1.4', 	'a', 	10) 
, ('2020.1.5', 	'a', 	10) 
, ('2020.1.6', 	'b', 	10) 
6 rows affected

select user_id
, min([date]) as start_date
, max([date]) as end_date
from
(
  select t1.user_id, t1.[date], t1.phase
  , count(t2.phase) as grp
  from your_table t1
  left join your_table t2
    on t2.user_id = t1.user_id
   and t2.phase = 'b'
   and t2.[date] >= t1.[date]
  group by t1.user_id, t1.[date], t1.phase
) q
group by user_id, grp
order by user_id, start_date


user_id
start_date
end_date


10
2020-01-01
2020-01-03

10
2020-01-04
2020-01-06