You should be able to get an idea by comparing snapshots at https://bitnodes.21.co/api/

They scan the network every now and then and list reachable nodes.  By checking various snapshots and comparing IPs you might get a sense of how often nodes are available at specified ips.  The only issue is that you may get nodes that stay-up but change IPs.  So technically you'd be counting nodes that reboot/go offline and/or change ips or some weird combination of that.

You can also look at their leaderboard:
https://bitnodes.21.co/nodes/leaderboard/

You'd be interested in DU, WU and MU which is the daily/weekly/monthly uptime of nodes by IP.   The top nodes on their leaderboard are all sitting pretty much at 99% uptime on monthly basis.