I´m sure that two hrs is enough but in order to develop even more you should play some more during vacation and than when work or school starts get down again to 2 hrs. keepin´ up and developing new stuff you learned during the increase time.

I started playn´ when I was 15 and played like 1-2 hrs but increased it all the time. Now I play 4 hrs. evryday and has been doin´ that for a long time. Somtimes I play 3 but mostly 4 wich I find enough. However I never end up playn´ 8 hrs day. I don´t think it´s good for your hands to sudenlly increase that much but that´s only my opinion do.

One more thing I realized that lot of you guys like to practice chromatic scale. I love that one too and it seems to be like most popular scale around. (I bet it´s only for us shredders).