To my kowledge, DNet might reissue blocks after 90 days. It is somewhere in their FAQs.
It is not certain if they will do so, though. They will probably begin with issuing the next key range, before reissuing.
To be on the safe side, take a time limit of two months.
Don't know where the one month time limit is coming from, haven't read or heard anything about it.
The only thing you might come across is someone doing random blocks and turning in some of the blocks you've been cracking but saving up. This is the biggest danger, IMO. This doesn't mean most of your blocks will be done by someone else, so you can still save. Normally, with a flush over a two month period, there is a loss of about 3-5 %.
Any time you save blocks you run the risk of not getting credit for them. The most efficient way is to send them in as soon as possible. When you crack a block & let it sit 90 days before you send it in that's 90 days that someone doing randoms might happen crack & return it before you do.