Hmmm, ok so the fondant (sugarpaste) ones I made I left for 7 days and they still broke with not much pressure.
I was adding tylose to fondant, not to gumpaste. I thought fondant + tylose = gumpaste but someone else has now said noooo. So I *think* fondant + tylose = modelling paste which holds it's shape but doesn't set up really hard (and therefore breaks). Flower paste sets up much harder, I'm going to try that when I have a bit of time to experiment.... needs to be soon before the xmas rush!
Thanks for all the replies, it's both interesting and confusing :D