I am going to do some maintenance on our exchange server over the weekend, and will be out of service for a good 12-14hrs.  I was wondering if there's a time or space limitation when it comes to queueing up e-mails on the spamfilter.  Thanks in advance.

Version of our spamfilter

Registered v3.5.3.678 

Desperado is correct. Please note that by default the "ExpireRetryQueueHours" is set to 0, which indicates "forever". So by default SpamFilter will keep trying to deliver emails to your SMTP server indefinetly until it comes back online.

We made this parameter editable because:
If an admin performs work on their SMTP server for just a few hours, it makes sense for SpamFilter to queue up the emails and deliver them when the work is complete. The sender will only see a few hours delay in delivering, so should not be a problem.
However, if the server is going to be down for 3-4 days, the emails will be queued for 3-4 days. The sender will assume they were delivered, as there will not be any non-delivery returns emails, but in reality they are still queued... this could be a problem if the emails were time-sensitive. If your SMTP server is going to be down for 3-4 days, it would be wise to configure SpamFilter to reject emails so that the sender does receive a non-delivery error email notifying them of the non-delivery (of course you could also shut down SpamFilter to obtain the same result).