I have had this computer for about a year.
How long have you been at this school?
I haven't seen Julia since September.
We use 'for' with a period of time, for example: a few days, half an hour, two years.
I've been at school since 8.30 a.m.
We use 'since' with the time when the action started, for example: last year, June 8, I met you.
I've been at school for 5 hours.