It really depends on the configuration. By default, the AP will reboot once an hour if it loses connection with smartzone but that is configurable / disablea-able. It will also depend what features you're using.
If you're doing captive portals then you might have a problem. Or radius proxy to smartzone. Or DPSK. But for " regular" wlans with PSK auth or even "direct from AP" radius, it will continue to work without smartzone. I run around 500 APs with a non-redundant (Google cloud hosted) vSZ. Never had any problems with losing services when doing system upgrades, etc. Also had some APs that were smartzone managed and used them disconnected from the internet (and thus SZ) for a while (several days) without issues.
Your mileage will vary and depends on the features. But there is a whole lot of survaibaility in smartzone APs.
