Done it for weeks on end
I went to Saudi Arabia in August 00 through Nov 00. The truck was plugged in from early October till I got home cuz the wife didn't want to drive it. I told her she had to drive it at least 50 miles a week, (one round-trip to her job) but she told me afterward she only started and unplugged it twice during that time frame. Everything worked fine when I got home so IMO, the block heater isn't going to wear out anytime soon.
-Adam