This depends on one hand on the story you want to tell, and on the other hand on your ability to provide enough content and variety to keep the game from becoming tedious. I've played 30 minute rpgs that felt too long and I've played 80 hour rpgs that left me wanting more.
For free games, short and sweet beats long and drawn-out anytime. If it's your first rpg, 1 hour is more than enough, although you should of course feel free to make it longer if you can't fit all of your (good) ideas into 1 hour.
For paid games, I expect the length of the game to be in proportion to what I paid for it, so a full-price game should have a full-price length and a shorter game should be cheaper. A full-price indie game usually costs in the $10-$20 range and a full-length rpg usually has 20-40 hours of gameplay so aim for at least 2 hours of gameplay per dollar of price.
