Recently I contacted an important researcher in my field, and he offered me a collaboration with his group. He told me to email him a proposal. Moreover, he recognized that he was usually very busy and received a lot of email, so I should be persistent. He even gave me his personal phone number.
I sent him the proposal, and alerted him by an SMS. He replied that he would look at it. It's been a couple of weeks, so I SMS him again, and he replied that he hadn't had the time yet to look at the proposal, but that he would soon.
This is a very important and unique opportunity for me. In view of his advice to be persistent, and the fact that he is very busy, I want to do what I can to remind him of this, until he acts on it.
What is the most polite way to insist? What time window should I wait before reminding him again?