How long would an I-765 be valid for if I apply for one while my EB-5 application is pending?

Usually EAD and AP are valid for 2 years as part of a pending I-485 if you request this. You may maintain your H-1B as well if you are still working for the H-1B employer. If you go unemployed you are no longer maintaining H status, but if the employer is willing, you can leave and re-enter in H status with a valid H visa.

Generally, the EADs are valid for two years and can be renewed as long as the application is pending. The H-1B can exist side by side as long as you are employed. It is a good idea to keep the H-1B since if the adjustment is denied, you can fall back on it. You need to have a specialist evaluate if the adjustment application is strong and the I-526 approval. I would not stop working on the H-1B, unless I was sure my I-526 is approvable. If you are not working for the H-1B employer, you cannot travel unless you have an approved advance parole.

Your EAD card will be valid for two years and can be renewed indefinitely so long as your I-485 application remains pending. You can file a renewal application 180 days prior to the expiration date of the EAD card. If you drop out of H-1B status, you can remain in the U.S. based on your pending I-485 application. You can choose to work using your EAD but you are not obligated to. Once you have your I-131 advance parole, you can travel outside of the U.S. and use it to return instead of your H-1B visa.