No longer than 4 months.
If you're putting out three updates a year then the game is never getting completed. Unless you're REALLY packing it in each update.
For me doesn't matter how much time they take as long as it is propotional to the new content. Short time with little content it's ok, but when it's been 9 months and the only new thing is a bad scene and 5 dialogues. I'm out.
Some developers can justify waiting 5 or even 6 months, but they are exceedingly few in number who can do so. Most however cannot be since their updates are like 2 scenes and last all of 5 minutes.