how long does it take to be a military engineer

Choosing the Military will take just as much energy and research as choosing a college. There a many avenues which can lead to you being an engineer in some capacity in the military.  

I encourage you to just not go on the answer I provide here but reach out to friends, family and other adults who may have some knowledge of the military to help you make a decision. Just remember that every person has certain bias and ultimately the decision is yours to make.

First what is your ultimate goal?  If you want to have a college degree and experience then maybe officer candidate would be your best option. Again there are several paths you can take here. Qualify for a military college, obtain a bachelors degree and enlist as an officer are two of the most prevalent paths.

Alternatively, maybe you want to go enlisted. The career paths as enlisted are probably going to be more specialized and focus on a specific aspect of engineering. So you should have a general ideal of what type of engineering you want to be:
Air Force: https://www.airforce.com/careers
Navy: https://www.navy.com/careers-benefits/careers/science-engineering
Army: https://www.goarmy.com/careers-and-jobs/mechanics-engineering?category=mechanics-engineering
Marines: https://www.marines.com/about-the-marine-corps/roles/military-occupational-specialty.html
Space Force: https://www.spaceforce.com/careers?gad_source=1&gclid=Cj0KCQiA3sq6BhD2ARIsAJ8MRwVsLj-iqv5X8n5Cg9Ohtxjzp5as3vEFWIum6C3KUmp-m79lv24oIxQaAixpEALw_wcB&gclsrc=aw.ds

Caution: When speaking to a recruiter, they have a job also. That is to get individuals into jobs/ careers based on the needs of that specific branch of service. Focus on what your goals are and be ready to walk away from this option if the options you are looking for are not open at the time you are looking to go in.

Don't wait until the last minute, many individuals obtain a slot in the career they choose through the Delayed Entry Program (DEP).  Individuals can enter this program prior to graduation or may choose to enter the program and go into the military at a later date