Each visa type has a different duration, and you can't study on a tourist visa.
I've had a visa that allowed me to work for 3+ years in the US, but was valid only for 90 days. Every time I left the country I had to renew it, a process which takes an indeterminate amount of time.
I've had a visa that allowed me to work for 3+ years in the US, but was valid only for 90 days. Every time I left the country I had to renew it, a process which takes an indeterminate amount of time.