For security purposes, each Masternode stake will have to wait until every node before it has received a reward. The wait time will depend on the amount of active MNs at the time and where the reward cycle is at the time of starting your Masternode. Rewards are issued every 10 minutes (For now; this may change in the future)
An Example: - Assuming there are 2500 registered nodes - 2500 x 10 minutes = 25,000 minutes - 25,000 minutes divided by 60 minutes = about 416 hours - 416 hours divided by 24 hours = about 17 days - In this case, there will be a 17-day delay from withdrawing your initial deposit
Please keep in mind that this is the "worst case scenario". Depending on where in the cycle the current reward is, it could happen sooner.
Another Example: - Assuming there are 2500 registered nodes - At the time of starting a node, the next reward in line goes to the 2401st node. (You only have to wait for the remaining 100 nodes to withdraw) - 100 x 10 minutes = 1000 minutes - 1000 minutes divided by 60 minutes = about 16 hours - Less than 1-day delay from withdrawing your initial deposit in this scenario
Basically, a full cycle needs to complete before you can withdraw your initial Masternode deposit.