Well firstly a few guidelines:
1) Don't go for more than 2-4 reps.
2) Don't use more than around 85% of your max unless you've got a spotter. In other words, you should be doing those 2-4 reps with weight you can handle easily for 6-10.
3) 3 seconds is about prime length for a pause. Any longer isn't really necessary, and shorter can still be beneficial.
4) If you have any doubts for God's sake use a spotter.