It varies drastically I've found and every song may be different. Having skype, forum pm's, or emails you can shoot back and forth is essential. Dropbox (best for sharing and updating the same stems) or mediafire for immediate file sharing. If you're using the same DAw and vsts thats awesome but it can be hard if you're both musicians that does make it difficult. Having a vocalist do the vocal part and a musician the remainder is far easier. Some collabs went on for close to a year (Romantic Tragedy, my collab with Injustrial
) while others were finished in just a few weeks (Breakfast in Bed, Be Assertive). Each is different from the last in some way.
Communication is extremely important. Funny thing is how many people will do a collab and then never want to again (buck and run i guess) and some you may collab with time and time again. With the Maressey project i gave some of the more well known vocalists extra time as they appeared to need it..... some came through and others have put me off for months and likely won't get me anything in the end (ie we only seem to playing skype tag). Giving them too few reminders can backfire as musicians often just do not juggle time well.
If you have a good rapport that's a much better chance of success but if they love using the "do not disturb buton", beware. But others were much more immediate, went to work immediately, and were willing to redo sections that were off. Try to work with those people as tbh I'd rather work with someone less well known who clearly has the skills and talent and is making the effort then someone who's always unavailable and busy but popular (although EQD may say differently). One last thing is be sure you are both happy before you release the track, you can't undo an unrelease and if the mix or some element of it is underwhelming then the damage is done....