How long to wait before cleaning out a child's room after he leaves home varies from parent to parent, and often depends on the circumstances and personality of said child. Some never clean out the room, either because they don't really need to or because they can't bring themselves to acknowledge that the last child has left the nest.
Other parents waste no time in converting that space to a sewing room, game room, TV room, or whatever their choice. Some have been known to do so before the child's car leaves the driveway!
The factors in play seem to be a mix of parenting style, relationship quality, and available space. Whether it's the first child to go off to college or the last to finally head into adulthood, parents are all different, just as their reasons vary.
If the parents have known and expected this day to come, and communicated that to the child, they may have more plans and be ready to proceed. They may have already discussed and budgeted for a new space of their choosing. Most parents realize that kids will leave at some point, but some are more ready than others.
The parent/child relationship is complex and ongoing; however, many parents are heartily sick of teen antics at this point and literally cannot wait till that child leaves home. Other parents may be so close to their child that they can't bear the thought of him leaving; perhaps even unhealthily so.
Since it is commonly accepted and expected that children will leave home upon reaching adulthood, there really is no excuse for a great deal of crying and angst. There is plenty of time to prepare for this day - approximately 18 years' worth.
Often when a child leaves home, the parents have already downsized a bit, especially if other children have moved out prior to this. Possibly the best strategy is for parents to discuss how best to utilize the vacant room and to plan and prepare ahead of time; some may wish to wait a few months, just in case the child unexpectedly returns. Others may decide to box and store the child's possessions and move on from this point.
One other consideration involves the reason for the move-out itself: if a child is going off to college, he likely will not return for at least a semester; it may be wise to wait until that time has passed. If the child is moving into his "own" place, and is financially and emotionally prepared, it may be less likely that he return.
It really comes down to a simple matter of preferences and accounting for emotions and practicality. Parents know the child, the situation, and have their own lives as well - it's not all about the child, after all.