I was a dial up user as well. As
Venom mentioned, dial up connection charges are not determined on the usage, but on the total time of the connection, irrespective of how much you use it ( he worst thing about a dial-up other than the slowness).
As far as i remember, it was:
0.40+0.10 =0.50 Rs./min for timings
6:30 to 10:30.
And,
0.12+0.10 = 0.22Rs./min for rest of timings.
So, if you use internet regularly, might as well switch onto
BSNL dataone, it will cost roughly the same for you.(Just an adive

)
Savvy