Is there an old discussion thread somewhere maybe that discusses silver? I tend to buy silver on Turn 1 or 2 if I'm not sure what other cards will benefit me, and again on Turn 3/4/5 if I have $3 with nothing pressing in the 2/3$ slots. I assume you can't put off buying silver too late, even if you get some $2 terminals, and my first impulse if I open with non-silvers is to get a silver or two. Or maybe I should look through the councilroom openings lists as consideration for which cards may be important enough to delay silver...
Basically, in terms of Silver, you want to look at what your strategy is.
If it's a modified Big Money strategy in a Province game, you can't really go wrong buying Silver.
If you're buying some kind of Action-heavy strategy, you want to look at what Actions you need to get, and how much buying power those Actions will provide before your deck comes together. If they're $5 (or more), then you'll probably want a few Silvers, quickly. If the ones you want to buy a LOT of are $4 or less, you'll probably want only 1 or 2 Silvers. You can get away with fewer Silvers if your Actions provide you with some buying power.
You want to remember that it's very possible to sink any cards/action engine (whether single-card like Lab or multi-card like Village/Smithy) by buying lots of Silver. You're almost always going to have the opportunity to buy Silver, almost every turn. You can really inflate your deck quite quickly with it, and if you're trying to chain together +cards actions, it'll be easy to draw a bunch of Silver (and opening cards) instead.
Silver is worse in Colony games and better in Province games. It's better in games where you aren't going to try to chain together mega-buys.