As far as how long to fly that depends on what your rig is loaded with (gimbal,FPV, etc...) that varies from Phantom to Phantom and how you fly. You can experiment with just a full battery hover - that'll give you max time without any heavy load. The heavier the rig the less fly time.
We all fly third party batteries - some good, some not so good but will it do damage - most won't unless they are really generic junk. Months ago folks bought some cut rate Ebay specials and had issues with length of flights (really low) and not being able to charge after 6-7 flights. Search the forum for battery recommendations - personally I like the mad dog 2700 but that's me.
You don't have to let the quad sit between batteries but let batteries sit 15mins after flight before charging.
7 batteries in an outing shouldn't bother your quad. After each flight feel the motors, they shouldn't be hot to the touch.
"Pegasus I" - P I.5, E300, V2, Futaba, Zen 3D, FPV, IOSD, DVR, 2.4 G/S, GTU 10.
"Pegasus III - P II,Zen H3, 2.4 G/S,FPV
"Spook" - 450,zen H3,FPV,IOSD,E300,Futaba
"Copter-Mobi" - Kong 450 - E300,V2, Zen,IOSD,