Don't know which aircraft you are talking about, but there is precious little contract work in Europe on say A320/B737.
China & Middle and Far East seems to be where it is at.
If you are thinking of a certain Irish Airline with blue & yellow aircraft, yes it is common to have a month or more without work, but in any case you are paid per scheduled block hour & you will reach 900 hours long before the year is up anyhow. No fly no pay for the period you are "gardening", and rumour suggests that the tax man is coming, so best to regard the headline figure as gross rather than net if you want to avoid dissapointment.
Most peoples experiences of agencies seem reasonable, but they get the money from the airline not you, so their ultimate masters/loyalties lie there. An awful lot of new agencies have sprung up lately too, I would treat with suspicion unless you can have positive feedback from a few other bods.