While we do ask brands to respond within 2-3 business days, there could be a number of reasons why a specific brand hasn’t gotten back to you. They might have a number of internal projects going on, finishing up previous collaborations, on vacation, etc.
I always encourage our creators to send follow-up messages. The main thing is to be very genuine and specific. If you're excited about a certain product, tell them why and what kind of content you want to make for them! And don't get too discouraged - these guys get a lot of proposals, so you're not going to be able to get everyone, but definitely, put more detail and thought into the ones you really want and you'll have a better shot.