Can a dream be started directly after you fall asleep? Or was I actually asleep for hours and the time went by. [/b]
Yes, it can be started directly after you fall asleep....thats how people WILD without WBTB method.

If the dream I had (though it seemed to have lasted merely seconds) started immediatly after I fell asleep does that mean it was a WILD? [/b]
No. You have to stay conscious during transition process from waking life to dreamland, then it is considered as a complete WILD.