Have gutted the entire bathroom... added 2 floor joist side by side where the feet will be. Now I'm wondering if the travertine will crack. We have the casters for the feet... how long should it cure before putting weight on it?

Travertine is fairly soft but with casters to distribute the weight you should be OK. I'd wait 4-5 days for the thin set to harden before putting that much pressure on it.