256 MB single side memory modules can be either 7.8 us or 15.6 us, it depends on the number of rows in the chips: 4096 rows chips have a refresh rate of 15.6 us, 8192 rows chips a refresh rate of 7.8 us.
1 GB double side memory modules can be either 8192 rows (7.8 us) or 16384 rows. In this case often the chips refresh two rows in one refresh cycle, so the refresh rate is stil 7.8 us.
In the end the best way to know the refresh rate is to look at the spd of the module.