calculate current streak - sqlite

I hava table with sessions

sessions
-----------------
_id | date 
-----------------
1   | 2015-12-25
2   | 2015-12-24
3   | 2015-12-23
4   | 2015-12-22
5   | 2015-12-22
6   | 2015-12-09
7   | 2015-11-04
8   | 2015-05-11

Now how to calculate current consecutive streak, here in the above table
current streak is 22 dec- 25 dec -> 4 days

i can remove duplicate dates using distinct command

SELECT DISTINCT(date) FROM sessions ORDER BY date DESC;


  date 
  -----------------
  2015-12-25
  2015-12-24
  2015-12-23
  2015-12-22
  2015-12-09
  2015-11-04
  2015-05-11