Most salespeople follow up on a schedule: three days after sending, then a week, then a polite "just checking in". The schedule has nothing to do with the buyer. It's a guess dressed up as a process.
And the guess leans one way: late. A buyer who asked for a proposal is most curious when it arrives, and the first impression forms while they read it, with or without you. By day three you have often missed the moment they were thinking about you.
The window is the first visit
When a buyer opens a proposal for the first time they are, for a few minutes, fully on your problem. They have the price in front of them, the timeline, the objections forming. If you call within the hour, you are answering questions they are actually asking themselves. If you call on Thursday, you are asking them to re-load the whole thing into their head.
A follow-up is a question about something they just read. Otherwise it's an interruption.
This is why "opened" matters more than "delivered", and why time per page matters more than "opened". The event you want to react to is not the email landing. It's the person sitting on page 3 for three minutes.
What to say when you do call
Don't say "I saw you opened it." It's true, and it's creepy. Say what the page they lingered on is about:
- Pricing page. "I wanted to walk you through how the phasing works, because the number on its own looks bigger than it is."
- Timeline page. "Is the March start a problem? We can move the first milestone."
- Team or about page. "Do you want to meet the people who'd actually do the work?"
- Skipped to the end. "Would a one-page summary help you take this to your team?"
The second visit is the real signal
A first read can be curiosity. A second read, especially one that goes straight to the same page, is a decision being made. Someone is comparing, forwarding, or preparing to argue for you internally. That's the visit to drop everything for.
The schedule isn't useless. Keep it as a backstop for proposals nobody opens. But the moment you can see reading happen, the schedule stops being your follow-up strategy. The buyer's attention is.