You could wake up after around 4½ hours, then 6. That's when the REM is about to end or has just ended for most people. Before that, and you won't dream anything really. But waking up every 1-1½ hours by an alarm is actually very good if you want to recall alot. I usually go for one big WBTB, and plan it out a day or two in advance. I put alot of faith in my WBTB attempts, and I recommend you to do it too. They become even more effective if you do.

The normal tasks we've had so far, includes fragments if it's recall. But I guess you want to recall 5 actual dreams in a week. You're doing okay Fragments is better than nothing. One step closer!