I have an outdoor bonsai that I want to display indoors from time to time. How long can I keep an outdoor Bonsai inside....

You can have a tree indoors for two or three days only, then it needs to be out again

Its also more challenging in winter - as we want to make sure a dormant tree (say temperate trees, that are kept outdoors, as they should) stays dormant. So I wouldn't take dormant trees indoors at all, not even for a few hours. 

In summer its easier, and you can keep a tree indoors longer - especially when you can provide it with a spot with lots of light. I don't tend to keep my trees indoors at all - but I guess a few weeks wouldn't hurt?

As always with bonsai, the answer is.. It depends on the species and time of year. Tropical bonsai are fine being indoors for months at a time often.