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15 hours per game is indeed a problem. It would be great if a game lasted a maximum of 6 hours so that you could play it in the evening.
But even on a small time limit, it's not always possible to finish a large HL+ game quickly.
One thing is to play a serious game. But sometimes you need to step away from the computer. That's why I like chess clocks (used in TE-lite and VT versions), so you can step away during your turn without having to explain why you're taking so long.
Another thing is playing casually or just wanting to play a quick game (I usually call this "fo-fun"). Then I play on a 2-minute time limit in SOD or 600+300 in VT, which is not enough for a serious game.
Unfortunately, the trend is towards increasing the time limit. That's why there are more and more unfinished games in our tournaments.