How much time does it take to manage sites within a multisite architecture as compared to multiple single installs? A service my company offers is similar to managed hosting where we help keep a server, Drupal, related modules, and backups up to date on behalf of clients who can't or don't want to do that in house, and typically this averages out to be about 30 to 90 min of work per site per month.
A client has asked me to manage their entire portfolio of 70 Drupal sites, all of which are part of a multisite architecture with site files on one server and DB on another.
I can't imagine that this is 70 hours of work, but I'm not familiar enough with multisite installs of this size to understand fully what this would entail.
To ensure that I estimate time correctly, what would I need to take into consideration when comparing management solutions for single server / single site installations as compared to the same number of sites in a multisite installation?
Short: It really depends.

Long: Each individual multisite installation adds complexity to the whole. If these 70 sites are more ore less the same in terms of individual content but (nearly) same codebase, modules and similar themes (imagine a large organization spread over the world with region specific characteristics), you'll most likely need less time to maintain. 

If these are 70 profoundly individual sites, you'll probably end up checking dependencies over dependencies for modules and themes, debugging deadlocks and incompatabilities. Maintaining this can be a real challenge and 70 hours a month are gone faster than you have "drushed and rollbacked" everything to working.

Accurate documentation and a SLA designed fair for both parties should be the key to a successful maintenance.

(I maintain a multisite installation with around 30 instances for a large organization with all the pros described above.)