The simple answer is: never.
Follow up is the key to growing your business. When and how you do it is up to you but you can always do it. There’s no time limit or statute of limitations on follow up. It’s better to do sooner rather than later, but it’s never too late (unless your prospect is no longer with us, and even then, the project may still be alive).
The amount of time that has passed will simply determine what you say and how you say it. If it’s been a while, you’ll need to remind them who you are and what you talked about.
For example, if you met someone last month at a luncheon and haven’t followed up yet, there’s no need to feel bad or to apologize. Simply say, "I can’t believe it’s already been a month since we met. I’m finally catching up on my follow up and wanted to be sure and let you know how much I enjoyed our conversation, etc."
Even if you met someone last year, you can still follow up – it’s just that you may need to remind them who you are. Say something like, "I just found your card buried under a stack of papers from last year and I hope you remember me. We met at the NIRI Conference and I was wearing the bright orange shirt and we talked about chocolate and the Scarsdale Diet."
This is where those notes on the back of the card come in handy, as well as the strategy that has you wearing something unusual that you will be able to refer to perhaps forever (if it’s unusual enough).
And notice that there is no apology anywhere. Unless you promised to get back to them with some information and didn’t (in which case, it is appropriate to apologize), you really don’t need to say you’re sorry. After all, they haven’t followed up with you either.
And even if they have, we are so barraged with messages that people are much more forgiving if one slips through the cracks. Your job in reaching out is to fill in those cracks.