It's not a matter of how far you ride, it's HOW you ride.
Tooling around town is only going to get you in the low-mid ranges. You want to do highway riding, with clean, fresh plugs and ride about 5-10 miles. Change speeds during this ride, and include accelerating onto the highway and drive like you normally drive. If you include some 35-45 mile an hour back roads, then fine.
I've read methods where theoretically, you need to ride a few miles, then chop the throttle at speed, pull the clutch, turn the engine off, pull over and check the plugs THEN, not tooling back home and riding slow though town again, loading the plugs up.
Personally.....I've never been a fan of pulling steel plugs out of hot aluminum heads.
I check my plugs about once every 2-3 months. I like to pull them, look at the color and if it's OK, then you're generally fine, unless a drivability issue arises or you notice something "different", like exhaust tone, popping or lugging. then the plugs are the best way to see what's going on.
Never rely on plug readings if the bike sat and idled for a bit, or if the bike ran short, slow ride lately, around town, or especially after start-up with an enrichener. The readings will always show richer then normal due to the added fuel.
“When the going gets weird, the weird turn professional....” ~ Hunter S. Thompson~
R.I.P. Rubberdown. 1971-2012