How long can I use a macbook that needs battery service?

Unless the battery is bloated or inflated in any way you will be able to hold off on the service for as long as you would like. You may notice that you Mac no longer lasts the fun day without being charged but as long as you keep a charger with you, you shouldn't face any issues.

If the battery is bloated or inflated (signs of this are warping of the keyboard or the bottom of the Mac) you will need to shut it down and take it to an apple store or authorised service centre to have it looked at. They will be able to tell you if it needs immediate attention without any cost and will quote you on the cost if it does require immediate attention.

Please don't worry about it.

Use it on battery when necessary and plug it in whenever possible.

About battery cycles

https://support.apple.com/en-us/HT201585

Related:

1. Update the software on your Mac,

    Update may come with bug fixes.

    https://support.apple.com/en-us/HT201541

2. If your Mac battery runs out of charge quickly

3. Battery Management.

   How battery health management helps

   The battery health management feature in macOS 10.15.5 is designed to improve

your battery's lifespan by reducing the rate at which it chemically ages. 

   The feature does this by monitoring your battery's temperature history and its charging patterns.

   Based on the measurements that it collects, battery health management may

 reduce your battery's maximum charge when in this mode.

   This happens as needed to ensure that your battery charges to a level

that's optimized for your usage—reducing wear on the battery, and slowing its chemical aging.

   Battery health management also uses the measurements to calculate when your battery needs service.

   https://support.apple.com/en-us/HT211094?cid=mc-ols-energy_saver-article_ht211094-macos_ui-04022020

See also:

Monitor your Mac notebook computer’s battery

If your Mac battery won’t charge

If your Mac battery status is “Not Charging”

macbookuser72 wrote:

My current cycle charge is 138. I have a macbook air that i purchased brand new last August from the apple store. I have warranty however I was told that it will not cover me for batteries that need replacement due to usage/wear out.....

If you have been getting a "Battery needs service" message, is still under warranty, you should be able to get it repaired at no cost as the rated lifetime of the current batteries is 1000 charge cycles and yours is only 138. You are definitely no where near "wear out". Plus the message can also be some battery related issue and not just the battery itself. There is something wrong with the machine otherwise you would not get the message.

Call Apple support directly on the matter and if you get the same answer ask for your case to get elevated. I would be insistent but in a diplomatic way.

You best get a repair ticket started to have the battery replaced - use the link in the upper right corner of this page.