I have minis that have been in the same 12" pots for 2-3 years. Should I remove them and replant with fresh potting mix? If so, what time of year is best to do this?
The common rule of thumb is fresh potting mix every three years. I repot mine any time of the year except the heat of summer and dead of winter. You may also want to step up the size of the pot so they have more room to grow. Even my mini that stay about 1' to 2' high are in 14/15" dia pots.
Make sure you try hard not to break up the root ball and the rose will never know what hit it. I repotted 4 this year and they never skipped a beat.