You can make an offer for lounge access at any time, even on the day of your travel. However, it's generally a good idea to make your offer as far in advance as possible, especially if you're traveling during peak periods or on holidays. This gives the airport lounge plenty of time to review and consider your offer, and it also increases the chances that your offer will be accepted.
It's worth noting that some airport lounges may have stricter policies when it comes to accepting offers. Some lounges may only accept offers made a certain number of days in advance, while others may have different requirements. These are internal lounge policies, and we have no control over them.
Overall, the earlier you make your offer, the more time you'll have to negotiate and the better your chances of getting a great deal on lounge access. So, if you're planning to travel soon, don't wait until the last minute to make your offer - get it in as early as you can!