Depends on the plug really, & how hard to access.
On the SYM it's pretty easy to get to, & cheap so I tend to replace it when I pop it open.
For the burger, they're a PITA to get to, so I use fancy ones and don't check 'em real often. Cuz I use fancy ones, that cost a fair bit more, & last really well (same plugs in my RAV4 go 120k according to Toyota) they were as new, so I just put 'em back in last time I had it open for a nightmar..er I mean Burgman valve check.