I don't think there is an definite answer to as how long it is to be "too long" to be single. It stems down to a personal choice for each individual. For one, taking a couple of years would set his/her pace to get settled but for another, it could take couple of months to get back on the track.
Just because one moves on a little bit faster than another does not mean that the previous relationship is a joke. It just means that some are able to heal faster than another in an appropriate time frame.
But yeah, the longer you are out of a relationship, it will be a bit rusty to get back in on the track but chances are, you'll be able to get the hang of it. 
