I've seen a thread or two on this if you want to dig for a more in depth discussion. The most basic requirement I've seen is keep your hands on the bar. If you're resting for a minute, getting a drink, and chalking up between reps you're doing singles.
Don't worry about set up for the first rep since you haven't started yet, that time doesn't count. 15-20 seconds between reps... I wouldn't worry much about that either. But use your judgment. If you're starting to think you're edging into 5 singles versus a set of 5, and what you intend to be doing is a set of 5, drop a little weight so that you can knock them out faster.
There is a current form check where the OP asks Rip if his pause between DL reps is too long. Rip's response was that the pause wasn't an issue per se (at least in comparison to a back rounding problem) but that it may be indicative that the lifter is using a little more weight than he should be for a legit set of 5.