It depends on alot fo factors:
If you've heard this phrase from mcafee, they are exactly right- "it depends"
I can offer you a suggestion- create a scan template with all of the options that you will be using to scan most of your systems. Then, create a scan and point it at a good sample population of the hosts you intend to scan regualrly. Perhaps you want to group hosts by operating system so you can get a decent metric of how long it takes typically per host