That’s not a good idea, even apart from generally not being a good idea, they would just have to ask support and would know, and, what’s more, support would know that you told them that.
With some people, it can work trying a compromise and giving a good and actual reason that anyone should be able to understand, like “I’m sorry, but since I need to schedule my time and factor in eventual revisions for all my current and future orders, I can’t let orders stay open for such long periods. Fiverr’s period for buyers to check their deliverables and ask for a revision with actionable requests as to what they want revised, is three days. Seeing that your order is bigger, those three days may have been a little short, and I’m happy to give you another week of time to check and get back to me with your list of revisions if any.”
Just an example, and reasoning doesn’t work with all types of people, but some are open for actual arguments if they are presented well and nicely, without a reproachful tone or such.
Won’t work with someone who purposely tries to stall things, of course, just because they think they can, or with those who are in truth scheming for a cancellation by using such tactics.