I was just wondering how long should I breastfe...

I was just wondering how long should I breastfeed for? my 4mnth old feeds for at least 20 to 30mins, 10 to 15mins each side.... Should I be feeding her longer? When I think she's finished, she always seems to put her fingers and hand in her mouth like she is still hungry! I do take note of the time when she starts to feed, being it 10 to 15mins each side... So I just wanna know if I'm feeding her long enough, how long should she feed for?