I echo the ‘take any offer’ comment.
How long is completely dependent upon… YOU.
You will not be a solo watch stander any time soon. It will take several things to come together. ( a calendar is NOT one of them)
You must have the luck (sorry. It is luck) to get with a Captain who will allow you to gain experience to get a recommendation. Most Captains won’t bother (especially in this downturn). Most importantly it is YOUR aptitude and attitude that will allow you to matriculate up the scale. It’s not what YOU think your skills are. It’s what the officer who can recommend you to the office thinks.
Office HR will spill all sorts of BS about how soon, won’t take long, or some other crap just to fill a slot.
You don’t need an unlimited license on an ATB (in the wheelhouse). Some companies may desire the unlimited license. But that is company BS versus reality on actual experience needed.
You need boat handling experience. That wasn’t gained at school. It wasn’t gained on Swim Team. It wasn’t gained while on a 90 day sea term and most importantly it wasn’t gained on simulator time.
It will be gained by actually grabbing the helm and showing someone (who will vouch for you) that you can do the job. (IF you can find someone who will let you drive).
On to a more narrow response:
Being a ATB mate is akin to being qualified to being Master. Tugs work differently than ships. Tugs work with a Master and a Mate.
Due to USCG regulations we are limited to working 12 hours a day. This means if I am working MY 12 hour watch my Mate is working the other 12. So, in effect on a tug the Mate must be just about as qualified as the Master.
If My Mate cannot make decisions or ‘stand his own watch’ that means I have to come up. Which means I have violated the 12 hour rule. NO company or competent master will allow you to assume a watch unless you are proven, have shown, and have a complete TOAR.
It is WAY more complicated than this. But there is a difference between ‘having a license’ and being Competent. Knowing the difference is what will separate you from the other shiny new alumni. The actual skill needed is minimal. It is the understanding of what is needed that is tough to comprehend.