How do you know when it’s time to let go?

If they do not wish for you to be a part of their life. If you feel like there's nothing you can do that would make it better. If they don't want to have anything to do with you, then you have to respect that and let them go unfortunately :( i know it's though, just went through it with my ex i loved so much

It's not fair to you or them, to go on when you don't feel the same way anymore, simply trailing them along.

“Some people believe holding on and hanging in there are signs of great strength. However, there are times when it takes much more strength to know when to let go and then do it.” ― Ann Landers

The fact that you've posted this question with the wording "once loved" means you've pretty much answered it yourself. Most people know with a gut feeling that a relationship is over, some realise that they're just not making each other happy any more (or are not happy themselves).So to answer the "what makes you realise?" question - many things, it will change from individual to individual, but in the end you'll know if you're happy or not, and if the flames can rekindle or not.