How long is a piece of string? This varies enormously depending on the species, and of course when the spider last ate, and how big its meal was.
The average life-span for most species is about a year, if dying of old age. Some can stretch to two years if they get somewhere warm to live over the winter.
Longest living are female tarantulas, who can clock up twenty years.
Further to last answer - because spiders need live prey to survivev, they are adapted to making the most of their food sources, and their stomachs will expand to accomodate the spider's own weight in food.
Some species can last up to a year without food, but as a norm, a few days without fresh food will see most of them off.
Thanks for the answers folks, I realise it may have been a bit vague. Hopefully the spider currently residing on my ceiling will have had a big meal and have enough strength left to find it's way back out the window!