Any time after the last frost is good and the sooner the better. Traditionally Valentine’s Day is considered the day to prune your roses.
These are young roses. I planted them less than two years ago when we moved down here. They have done very well. I have not pruned them heavy. I wanted to give them a chance to settle in some first.
You need to cut off the dead branches first. Then starting at the bottom remove any branches growing in towards the center. Then the branches that are thin or weak. Then any branches crossing other branches. You should end up with a bowl shape to the plant.
I also cut the tallest branches back by at least half each year.
If you prune your roses back to about a foot tall you will get fewer but larger roses. If you prune your roses back to about two feet, you’ll get more roses but they won’t be as large.
A good rule of thumb on how far back to cut your roses to cut as low as the first group of 5 leaves. Roses grow out in groups of leaflets. Find the lowest group of 5 and cut to about 6″ above that leaflet.