Long enough to get comfortable

.
I would sit a gamer in front of it, and see if they comment - it's the only way to know for sure is to try it out. If they ask if it's crashed then it's too long.
There are things you could do though...
* Set the sync rate to 0 when initializing and loading up media.
* Use .DBO files instead of .X - they load faster.
* Clone or instance objects whenever you can, instead of loading them up - it's quite easy to do if you load objects in for...next loops.
* Show a progress bar - like work out how much video memory your game takes up and measure that to get a percentage complete value - seeing a number go up slowly is better than no numbers.
* Load all your media before doing anything with it - the more sync's you do when loading media, the slower it gets, as models are positioned at 0,0,0 by default.
* Turn off autocam (AUTOCAM OFF), this will avoid issues with creating new objects and sync.
* Reduce the render size of the 3D output (SET CAMERA VIEW). This can help as the smaller the render area, the faster it renders - so even if you do end up syncing with unnecessary objects on the screen, it won't kill performance so badly.