As you send quotations, it is important to set a quotation deadline, both to entice your customer into action with the fear of missing out on an offer and to protect yourself. You don't want to have to fulfill an order at a price that is no longer cost effective for you.
Set a deadline
On every quotation or sales order you can add an Expiration Date.
Use deadline in templates
You can also set a default deadline in a Quotation Template. Each time that template is used in a quotation, that deadline is applied. You can find more info about quotation templates here.
On your customer side, they will see this: