you can play as long as you wish,if you can bear the pain,nothing wrong with playing 10 minutes or one hour straight.
a)At first you wont be able to do chord progressions quickly,so try to fret the chords at first and once in a while try to do chord progressions.
b)you can use whatever finger you like,but
more alternate finger positions=much easier/better/economical chord progression
for example if you play an E minor using fingers 3,4 [instead of 1,2 0r 2,3] and the next chord after E minor is an F major,then you dont have to lift your fingers,you just have to slide the fingers 3,4 on to the 3rd fret[string A & D] to make an F major plus other fingers.In the same scenario,if the next chord after E minor is a G major, then the more economical fingering for E minor will be 1,2,since you dont have to lift the finger 1 to do a transformation to chord G major.[hope you understood]
I dont know how long youre playing,but if youre just starting out its better to learn a fixed fingering for a specific chord and after some time when you learn most of the chords,you can just start doing alternate fingerings,though there is nothing wrong in trying now
Thats too bad
Thanks. I got the part about the finger positions and chord transformations. I currently practice for about an hour-long session, 3~4 times a day. Hopefully once the fingertips become harder, I will be able to play at a stretch instead of stopping every alternate minute.