The players only make skill rolls if there is something at risk, if it means something. What that means can of course differ from adventure to adventure, session to session. Our sessions are about 3-4 hours long and my players have earned (through skill rolls) anywhere from none to a handful, two or three being the average. The main reason for this is that not all characters are making all the skill rolls in every situation and even with skills that usually a lot of players roll for if you earn a mark in a skill you can't earn more marks in that skill until the next session.
When it comes to the questions, well how to interpret this will be up to the GM but in our group:
Did you participate in the game session?
That's one.
Did you explore a new location?
This doesn't happen every sessions since the PCs aren't exploring adventure locations every session and when they do explore one, it may take more than one session before the adventure location is considered explored.
Did you defeat one or more dangerous adversaries?
Not all NPCs/monsters are dangerous. A combat between the PCs and some highwaymen aren't that dangerous if the PCs easily win the fight. A monster that was killed in one round was clearly not that dangerous. What dangerous means can differ from group to group. A group of travelling merchants and scholars may be in some serious danger if facing some ordinary highwaymen.
Did you overcome an obstacle without using force?
The obstacle in questions here must be of a very dangerous natur otherwise it would be comical. Jumping over a stream is not something that should be worth a mark.
Did you give in to your weakness (optional rule)?
In order to earn a mark for this the PC must face his weakness. It must mean something. How often a PC will have the opportunity to do this depends on the weakness and what adventure you play.
In our group, the PCs usually earned two of these five, three if they managed to play out their weakness, one more during those sessions when they finish exploring an adventure site.
The PCs in my group have at most earned about seven marks in a session, but that's rare, most the time, they will earn a handful. I also don't allow exploits like rolling for a skill just to get a chance to earn mark, a skill roll must mean something. To face your weakness must really be something that has an impact for the character, not just that a PC take one silver more than the others to fulfill the Greedy weakness.
But every GM and group will do what fits them best, there is no right or wrong. If the PCs in your group earn so many marks each session, they will also become more experienced fast, so you may have to adjust for that. Just as you may have to adapt if you only play one hour sessions by giving free marks and roll for advancements every two or three sessions. If you play for very long, you might want to divide your game night into two sessions. When we played FbL we had very long sessions (6-8 hours) and we divided them into two sessions each game night (one before and one after food break) for experience reasons.