DiRT will not have a problem with that much data - we've done test backups and restores on much larger systems.
As for how long it will take, how long is a string?
There is no snappy answer for this since it depends on so much external to DiRT. The restore in particular depends entirely on the speed of the AD lookups/binds which varries wildly from site to site. The message backup and restore uses ExMerge from MS which takes each message from a user's inbox and stored it as a PST file. This causes single instance messaging to be lost and the actual size of the bacup and restore can be quite different depending on the number of group messages and such. And, of course, the speed of the Exchange mailstore and network weighs in as well.
I've seen backups in our labs of roughly the same size systems vary by as much as %300 based on traffic and configuration.
I wish there were a snappy answer I could give you but there's really not. The backup can be done any time and will run fairly quickly other than the message backup. The DB backup and local file backup will not take more than 10 minutes in most cases. The message backup (if you do that) is not predictable - one of the reasons it's optional.
The restore will be the lion's share of the time because it needs to find each and every user in the directory and update its AD info or create new AD records for each user. This is the vast majority of the time for a restore and is completely dependant on your environement.
