How long do I wait for a response from a prof?

[fingerscrossedSLP]

First time grad applicant so bear with me lol. I've sent an email to two profs asking for letters of recommendation almost 3 weeks ago. The first one responded the same day only a few hours later, but the second one hasn't given me anything in response. Maybe it's too early but I'm not sure if I should assume that he's not interested, if I should send a follow up, or move onto another potential professor. And if I do go with someone else, I'm not sure what I would say if the no-response prof does decide to reply. Thoughts? Should I calm down and wait longer? Thanks!

*Also, can confirm that I have the right email address and he definitely knows who I am since I added him on LinkedIn not too long ago and I took his class last semester.

Edited November 13, 2020 by fingerscrossedSLP

[Sigaba]

Unfortunately, uncommunicative professors is a recurring theme, season after season. You may find field/discipline specific guidance in the SLP forum but if you're willing to dig in this forum, you may find guidance that is useful, if not exactly comforting.

FWIW, I recommend that you follow your instinct to "calm down and wait longer."