I get them to sign a contract that includesMy language with schedule days / durations in it4.

If someone outbids you and you cant outbid him, then he becomes the winner of the property and from what he gives them they return your money, so no chance to loose your money, but it is tied up for the duration of the bid process.

In addition to scope of work you should walk them thru your expectations, job site clean up, safety, handling materials, assisting in take offs if needed, recommendations, work hours, crew size, duration, schedule, payment, payment process, warranty, equipment operation, etc... 

Commercial financing has limited duration fix. 

Is there a specific time duration in which sellers should respond to offers?

The notable exception is fix-and-flip projects because the duration is so short.  

The loan duration for the land contract will be for 1 year, at equal  amount of payment per month. 

With a flip, your exposure is of short duration, so probabilities or risk are on your side.

Another requirement I've seen is that you must live in the house (as your primary residence) for the duration of the loan/until it's paid off.

Don't quote me on the duration as I am not sure how long they require it to be.