people are coming up with crazy numbers like 200 runs thats 90-100 hours of spamming unless the lock out is longer then thats gonna be thats much longer. you should be forced to get an items to even do the instance. that would kill spamming. and not easy a hell items like inferno taper, unmarked keystones & garuda items you need to start the fight.
EXAMPLE only:
ffxi you got beastmen seals to trade for orbs at different prices and each orb was a different level then you used that orb to start different instances in different areas, there there was kindred seal did the same purpose but had different fights and so on. that kinda system your not penalized or able to spam.

then you had bigger events that costed a hour galss item that lasted an hour & costed 1mill gil and you could make like copys of it to give to party members once you enter the instance your timer started after the tier ended you had a 3 day lock out.

the ARR dungeons need to be more then just a kill quest also, even wow had quest you can get inside a instance.