Lots of variables, like UPTme said. However, I think he misspoke on a couple of points. There is no "limit" to mil days. That's illegal. The airline has to give you the mil days you request.
They definitely don't have to pay you for them, so his point about cutting into guarantee is correct. In fact, most airlines DO NOT pay you for mil days (AA is one exception here. They let you drop up to 4 days and it doesn't affect your pay - pretty sure I have this right. AA guys can jump in if I'm wrong). So, with most situations, any airline trip/R-day you drop is going to mean a pay hit. Obviously you get paid at your ANG unit, so there is some offset.
The way things have been going lately, it's likely your guard pay is higher per day than most mid-low seniority positions at many airlines (unless you're pretty low ranking in the ANG).

So, to answer your question: Depends on the unit. A fighter unit usually needs at least 6 maybe 8 days a month with more on busy months or if you deploy. A cargo or tanker unit may only need a couple of days a month for proficiency or they may need a whole bunch if things are busy. There's a pretty good bit of flexibility in most situations (not always). If you want to work your tail off, you can fly a full airline schedule and then do military on your days off. That would obviously leave you with very few days off (maybe 5 or 6?). You could also drop 75% of you airline schedule and work mil when you would have been at the airline. Then you have the same days off as your civilian buds at the airline. It's kind of up to you (usually).