chasv77answered grow question 5 years ago As mentioned before the quality of your soil will determine when you add liquid nutes.
If you are running the Fox Farm Ocean Forrest soil, I have found it can last 4-6 weeks, depending on how fast your plant strain is using up the micro-nutes of the soil.
I have no experience with Happy Frog, but I've heard it is less hot. Any other brand I can't comment on, except general rule of thumb, 3-4 weeks.
If your leaves are nice and healthy looking after 4 weeks, I would stretch the no feeding policy to 6 weeks. Always check your plants color and shape for any clues it is ready to feed.