Wait five to seven business days before following up on a sent proposal. A proposal usually needs to travel further internally than a regular email. Someone has to read it, share it, and get initial reactions from the right people before they can respond.
If you agreed on a review timeline during the proposal meeting ("we'll have a decision by end of the week") don't count days, count on the date. Follow up the day after that deadline if you haven't heard anything. The 5-7 day window applies when no timeline was discussed; when one was, use it.
For larger proposals that require budget approval or sign-off from multiple people, the review cycle is longer. If you know the decision involves a committee or an executive, ask about the timeline rather than the decision itself: "Do you have a sense of when the team expects to make a call on this?" That framing respects their process without leaving the conversation open-ended on your side.