It depends on the goal you are after.
If you are using sled sprints to improve maximum power output, then sprints of about 6 seconds with a moderate load are appropriate. If that is your goal, you should use complete, or close to complete rest so that power output remains stable (or even increase) throughout all your sets.
If you are using sprints to build acceleration/speed then you should use a lighter load still for sets of no more than 6 seconds. Again, using significant rest periods is important.
If you are using sprints to improve metabolic conditioning you can either do...
1) Anaerobic power: this is what I explained earlier
2) Anaerobic capacity: either sets lasting about 12-15 seconds with 45-60 seconds of rest or ones lasting 6-9 seconds with 30-45 seconds of rest
3) Lactate tolerance: sets lasting either sets lasting 30-45 seconds with about 90-120 seconds of rest or sets of 20ish seconds with 45-60 seconds of rest