This SQL query I wrote for myself might help you. It's kind of slow and a bit hackish. I'm sorry it's so ugly - I am not very good at SQL. This is filtered by number of hands per day rather than length of session, but should give you a similar picture. It doesn't break it down the way you suggest, but I already had it and thought I'd share.
You may have to change your player_id, and you can also change any other data in there to fit the parameters you want, like number of days in the past and how many hands to define a long session.
select * from
(select date_trunc('day', handtimestamp) as "Date", sum(netamountwon)/100.0 as "Winnings", sum(1) as "Hands"
where player_id=1 and handtimestamp>(current_date - interval '50 days')
group by "Date" order by "Date") as dailyresults