Depends on the size of the space, generally the larger the longer.
Are backfill tenants looking at the market now and decided if they're going to give their 6-12 month notices on renewal options or if their is a better relocation opportunity? If so, you're probably already late to the market. If they are they small operations on month-to-month leases or with 0-60 day notices, you have time.
It's not uncommon for large industrial or office spaces to be marketed 2-3 years in advance of an upcoming vacancy/expiration.