my friend it not depend on that time that how much you should spend the time on facebook it's just depend on you that how much want to be spend on it because it's will never end if you will not sign out .
Continuously update your fb page with quality material. I think no limit for time on fb because may be if you spend 2 hours but share junk content then people will ignore & even you will loose your fb fans. Better content = better, more & more traffic to your site.
If your marketing task is busy, you can manage your time by delegating task to your friends or your colleague. I think about 2 or 3 hours per day and a break btw each hour will make you manage your facebook effectively
I've been visiting facebook everyday but I make sure that it won't affect my work, so I give facebook at least 20 minutes a day.. but if you have a business on facebook you have all the time it depends on you..
I Mostly spend 12-15 Hours a day in facebook. I still couldnt realize what makes me to spend soo much time in facebook.
Try to affiliate your fan-page with other related fan-page and ask the author of other fan-page to promote your fan-page by sending invition to all its members, try this if possible, it can help you to get fans quickly... to promote your product...