The schedules vary widely from company to company and even within each company. It can range from "day trips" where you are home (at least back to your domicile...whether you choose to live there is up to you) every night....to trips ranging from 2-5 days...to some of the supplemental and international freight carriers where you could be gone for 14-16 days at a time. I would guess the average days off per month would be between 12 and 14, with the range usually from a low of 10-11 to a high of up to 20-21.....but this is just a guess. Obviously the senior crewmemebers would get the most days off (usually...however sometimes some senior people bid for the higher time (pay) lines).
I don't think FOs have a tougher time of it. Usually the lines (rosters) are the same for Captains and FOs. Possibly at some companies, there may be more FOs on reserve, so you have less control over the types of trips you fly...but there is an equal chance that there are more Captain reserve lines. And this could vary from month to month as well. Just depends on the staffing at the time, and the amount of flying to be covered.