I think it used to be worse. it had the long individual lockout for each trial, but also had a really stupid lockout that applied to all 6 trials if you finished just one of them (like 2-3 hours iirc). So you couldn't even reasonably form a group to hit all 6 in a row without waiting hours in between.
Unfortunately, it has always been 3 days. As Xyroff mentioned above me, there was even the generic replay timer preventing you from doing all of the trials back-to-back. I definitely think this could use some reducing, but that's more out of a sense that I don't want to wait to re-run these. It'll balance in the long run, and the trials themselves do offer some awesome filler-piece rewards.
It was 2 hours between trials. I guess they expected it would take you that long to clear to the next trial NPC.
