How much time does it take to manage sites within a multisite architecture as compared to multiple single installs?
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.)