For an average user just maintaining their own razors, you may NEVER need to lap a new Norton, given honing is a couple-of-times-a-year event, at most. But if you're given to picking up eBay strays and orphans, like me, you find yourself spending significant time stroking them back to health.

Rather than a raw number, I'd suggest setting the stone on a glass table-top (carefully) and backlighting it. When you can see a gap, lap it. The 4k side is lots softer than the 8k side and will need attention more often.