Our system lets employees pick up shifts as long as the shift has not already started. We don’t have a way to set a cut off time on picking up shifts (for example, to set a trade/pickup deadline for a certain number of hours before a shift).
If you require that shift trades or pick ups be done a certain amount of time before the shift starts, you could post your policy regarding the trades deadline on the employee home page. You can also require manager approval for all trades and pickups to control the timing and approval of those.