Certainly not advised, but you could manually edit the timeout in the DB if you look at Dbo.DiscoveryProfiles and the column -[JobTimeout]
With such large discovery ranges, you increase the risk of causing Out-Of-Memory errors running the jobs. It would really be best to split them up.
Scanning a /16 is REALLY not advised. Doing so requires 65,000 devices to either respond or time out.
If most of those /16s are not densely populated then your discovery is going to be waiting for a whole bunch of multi-second timeouts before it can move to the next IP to scan.
If you have the IPAM module then I'd suggest putting all your subnets into there as /24s and letting it tell you which /24s to go scan later.
IPAM allows you to bulk import subnets so it is much easier to create all those lists of networks with a little notepad copy/paste/replace magic.