The standard nowadays for employers in the US and Canada is one to two pages. Why? Mainly because HR staff only have so much time. And because if an HR person senses you're rambling on about nothing, they'll stop reading anyway, so why waste your time?
Even if you have lengthy, excellent experience, it's a good idea to remember the words of Polonius in So while one to two pages is not really a do-or-die rule, try to stick to it. If the résumé is longer, read it through to see what you can remove entirely or make more concise. And don't force a fit into one or two pages by, say, bumping your font size down to 4-point and stretching your margins to 0.2 inches!
If you're doing a curriculum vitae (see: