Well, I see two aspects to a "slow burn".
First, it must burn. Something must be happening between the characters. Some games have no chemistry, no advancements, then sudden sex after 12 hours and consider themselves a slow burn, to which I disagree. This is somewhat covered by the "time passed" aspect but not wholly, since
second, it really is about progress. A game which starts with introducing you to a love interest, then puts in a screen "you got along well, and after intense dating after one year finally ..." cut to sex scene. Progress is made, a believable amount of time has passed, but since we did not see and feel the progress, it is worthless and still not a slow burn.
And yes, for the player (as opposed to the character) waiting time may easily exceed the "curious" area into the "annoying" area. But that's the price if you support (or pirate) games in development you have to accept. The only true measure from an artistic POV can be what happens in the finished product. (Note: I don't want to imply that you have to support every and any waiting time financially, but that it cannot matter from an in game story perspective)