Really depends A LOT on teh code you're writing and all, so theres no real good guide except to try it.
The only thing thats come in handy to me is to benchmark page build times, and also somenoe told me this once too:
Generally, as far as performance is concerned, on Linux with PHP, do as much as you think you can do without entirely killing the machine... then do some more, and you'll be about good.
