In the UK you are restricted to 900 hrs p.a or 100 in the last 28 days.
It depends on the operator. E.g a turboprop night mail only operator you'll get around 300 hrs p.a, but a scheduled domestic you'll either work your butt off or do nothing (i.e either 900 or 300), depending upon if they are under or overstaffed at the time. This appears to be how most of them operate.
Charter airlines tend to have large seasonal variations. In the winter when a large proportion of the crew training takes place, at a large training base you may only do 20 hrs / month, but in the summer you'll do 99.5 hrs !
Most airlines will contract you for a number of hours.