Does Visa Expiry Period had any thing to do with period of stay in US

[venkime]

hi all

i am going to US on F1 VISA, joining masters this fall .. in 2006, But, my VISA expiry date is SEP 2008 ; I have 2 years before my VISA expires. Do I need to get back to India soon after my MS ? cant i avail OPT ? Please help .. what is the discretion for the VISA expiry period, some are getting 5 years too !