Early enough to obtain current financial statements, correct any record discrepancies and resolve status or travel questions without pressure. Accounts for a completed period are rarely available immediately, and corporate or tax documents from elsewhere take longer still. Since no processing time can be promised, working backwards from an expiry date leaves no room when something takes longer than expected.
Decide about travel before the calendar decides for you
Trips abroad, a consular appointment and an approaching expiry can collide in ways that are difficult to unwind. Map the family's planned travel against the application steps and raise conflicts with counsel while they can still be moved. Canadian citizens generally need an E visa, so a consular appointment is a scheduled event with its own lead time, not something arranged in the final week.
Negotiate with the seller a long-stop date that leaves room for consular scheduling, because interview availability varies, and a seller who insists on closing within thirty days will force either an unconditional payment or a withdrawn deal. If the seller refuses escrow and insists on closing, the partners can still apply, but the money is then fully at risk in the ordinary sense and a refusal leaves them owning a U.S. business they cannot run in person.