According to the Mouse Guard corebook (1st ed), players turns should take up a sizable part of the gameplay. I agree with this - its there to balance out the lack of autonomy in the GM turn.
I’ve played 6 sessions with my group now, and players turns are almost universally short. My group doesn’t get many checks so they tend to get spend exclusively on recovery checks. If they do have some free actions and want to do a roleplay action (talking to a particular person etc) it tends to take <5 mins.
From a GMs perspective, players turn is an area I have the least time to prepare for - it really depends on where the mission ends up.
How can I make the player turns longer, more interesting and more satisfying for everyone?
You say your players don’t earn many checks – how many is that? Are your players risk-averse or have they embraced the joy of failing a lot? Do you twist a lot, or do you mostly hand out success with conditions?
What are their goals and beliefs about? Just about the mission of the day, or do they have interesting things they want to do – unfinished business, urgent personal matters, enemies that need their butt kicked, friends in trouble …?
Do mission goals usually get accomplished in the GM’s turn, or do you hand over to your players when things are about to get interesting?
I frequently would end my GM’s Turns before the patrol had accomplished their mission, which would force them to choose between their characters’ personal goals (sometimes Goals) and carrying out their duty as guardmice. This conflict of interests is at the heart of the game. You don’t want to hammer them in this fashion every session–they need the chance to recover and make preparations, too–but it’s an effective tactic for enriching the Players’ Turn.