If you don't put any roster requests in for RDO's, you should generally speaking, get 4 x 2 days off. Maybe 3 x 2 and 2 single days.
The more requests for specific DO's you put in, the higher chance you have of getting more single DO's and less pairs.
In all my years in QLink, the only times I get more single DO's than pairs, is because I've put quite a few requests in.
You generally get 8 requests per 28 day roster period. Why not just request 4 pairs of RDO's? A mate of mine used to bid for every weekend off and would get it 90% of the time!