Set time is defined as the amount of time needed before a joint can be carefully handled. 

Cement goes through three stages during it's working life-thin syrup stage, thick and doughy stage and solid stage.  During the thin syrup stage the pipe and fitting are placed together and the initial bond is being made between the pipe,  cement and fitting.  The second stage is where the solvents are starting to evaporate from the cement and the bonding process is continuing.  It is during this thick and doughy stage that the joint should not be moved.  At this point the joint is weak and movement could break the connection.  And, because the cement is too thick it won't be able to flow back into place to make the connection again.  The solid stage is where all the solvents in the cement have evaporated and the joint will be ready for handing and use. 

Set times given are estimates.  There are many variables that enter into the set times and the end user must make the final determination.