I have a RB532 with 64 Mb of ram running 2.9.46.

I have a fiber WAN connection right now at 20/20 mbps.

The CPU load on the Mikrotik is averaging at 55% and peaks to 87% (according to Cacti polling once a minute).

At what CPU load should I upgrade to faster hardware?

At 99.99% ...

At 99.99% ...

agreed, mine pegs at 100% most nights, no lockups, no slowdown thay I can tell.

indeed, when CPU load reaches 100% and almost never gets off that number - you have to upgrade, if only hits like 80% no worries at all.