The paint seems pretty durable so far.
...And I think it will continue to be, Brummie. That "paint for plastics' is really durable stuff, and seems to bond very well with the plastic substrate. It is also flexible enogh to bend with the fenders without cracking or flaking, so you should be set for some time now.
I think you were very smart to apply the paint to new, unused fenders. No worries about scratches or trapping dirt beneath the paint.
By any chance, did you paint the inside as well, or just spray the outside and let whatever overspray there might be go inside? did you mask off the bbrackets that hold the stuts?
My, it sure does look nice! I am...smitten; a gorgeous look! You can surely feel proud of your accomplishment.
All the best,