Calculated it out since I just leveled. Leveled off a dragon level so my exp was near minimum as well.
One full stamina bar and 3 gathers put me at 3551/5900 xp from doing only 2-1 and specials, which give about 134 xp per 12 stamina.
Using the stamina regen time, and we'll say gathering averages around 25 stamina each. Range for it seems to be 0-50.
I estimate it'll take around 210 stamina to level from this point, which is about 631 minutes, or 10 and a half hours.
Within 10 and a half hours, you will regen 5 gathers, which if averaged will net you around 125 extra stamina, so you're way over already. 
So let's say we get 3 gathers, for a total of around 75 stamina, which takes 6 hours.
This would reduce the remaining stamina needed from regen to about 6.75 hours instead of 10.5 hours, so unless I'm wasting stamina doing dragon missions, I should still be able to get 3 levels a day unless I get really unlucky with gathers.
Also keep in mind, 2-1 is FAR from being the most efficient xp map, so I can only conclude you're doing something WAAAAAY different if you can only gain a level per day some days.