Looking on the internet deeply has found these results:

told me to let you know once I was free is the most popular phrase on the web.

told me to let you know when I'm free 119,300,000 results on the web	told me to let you know once I was free 1178,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
He asked me to let you know he's concerned about your situation. And your mother decided it might be best for me to let you know the news. You wanted me to let you know Director Sterling is gone. You told me to let you know the moment Planet Druidia was in sight, sir. They asked me to let you know that they will be staying downtown at the mainline hotel. Anyway, she just wanted me to let you know everyone's running a little late and was wondering if you could make it at 1:30 instead. You told me to let you know when it was a quarter of. Lukas wanted me to let you know, he knows.	Lukas wanted me to let you know, he knows. He asked me to let you know he's concerned about your situation. You told me to let you know the moment Planet Druidia was in sight, sir. You wanted me to let you know Director Sterling is gone. Anyway, she just wanted me to let you know everyone's running a little late and was wondering if you could make it at 1:30 instead. You told me to let you know when it was a quarter of. They asked me to let you know that they will be staying downtown at the mainline hotel. And your mother decided it might be best for me to let you know the news.

told me to let you know when I'm free

119,300,000 results on the web

told me to let you know once I was free

1178,000,000 results on the web

More popular!

He asked me to let you know he's concerned about your situation.
And your mother decided it might be best for me to let you know the news.
You wanted me to let you know Director Sterling is gone.
You told me to let you know the moment Planet Druidia was in sight, sir.
They asked me to let you know that they will be staying downtown at the mainline hotel.
Anyway, she just wanted me to let you know everyone's running a little late and was wondering if you could make it at 1:30 instead.
You told me to let you know when it was a quarter of.
Lukas wanted me to let you know, he knows.

Lukas wanted me to let you know, he knows.
He asked me to let you know he's concerned about your situation.
You told me to let you know the moment Planet Druidia was in sight, sir.
You wanted me to let you know Director Sterling is gone.
Anyway, she just wanted me to let you know everyone's running a little late and was wondering if you could make it at 1:30 instead.
You told me to let you know when it was a quarter of.
They asked me to let you know that they will be staying downtown at the mainline hotel.
And your mother decided it might be best for me to let you know the news.