Short answer: If clients haven’t accepted their proposal within 24h, it’s a good idea to send a reminder (automated email).
Be sure not to spam but a reminder every ~24h is a good idea. If they still don’t respond, it may be a good idea to reach out over their provided contact information to make sure they’ve seen your proposal or have any questions.
If your client can't find their proposal link for whatever reason (ie. can't access email anymore), you can share the proposal link directly with them by following the steps here.