F/As are paid in the same way pilots are. They are paid their hourly rate per block hour, or per 'trip', however the airline determines 'hours'. Read the section on the JC main page about how pilots are paid, and it's the same for F/As.
As for the specific pay rate most majors are currently starting at about $17-$18 an hour. When I started at AA in 2001, stating rate was around $20 or $21, but that was before concessions. Current majors hiring right now are United, Continental, Southwest, JetBlue, and probably some others that I'm forgetting.
Flying 40 hours a week on a consistant basis is pretty unrealistic. While F/As are not covered per the FAA in same way pilots are for 30 in 7, 7 day, etc, depending on union contracts there may be block time limitations. Even if there are NO block time limitations, flying anything over 120 hours a month is realistically not possible. Even 120 hours a month is REALLY pushing it duty-wise. Not to mention fatigue-wise. 120 hours a month pretty much means no time off, no life. I once flew 113 hours once in a month at Eagle. Once. I never did it again. 70-80 hours a month is pretty comfortable, 90 is starting to push it. 40 hours a week is 120 hours a month, but it gets fatiguing REAL fast and I you want to have a life outside of work and not be a walking zombie you're not going to be able to keep that schedule for long.
I hope the above made sense, I'm very tired today and if I didn't make sense let me know and I'll answer tomorrow after I've had some sleep.