Depends on the dimensions of the 90g.
Silvers grow fast about 1-1.5" per month. One of my silvers that I raised from 2.5" was a slow grower and after a year it was only 14". But sbuse had his 3" silver grow to 24" in less then a year, these two examples are likely extremes. If your 90g has a 48x18" footprint and starting with a 5"er, I would say that in 6-8 months you'd better have the bigger tank ready to go.