Recovery Time through armpit incision
I do not think you will have a significantly longer recovery by choosing the armpit incision. You are definitely safer over the longterm under your muscle, but if you want less risk from the incision standpoint, I would recommend the in the crease incision for you. All the best.
What is the recovery time post breast augmentation if I was to have sub muscular implants inserted through the underarms area?
I do not believe there is a difference in recovery time with incision placements. Typically transax implants sit higher initially compared to the other placements - so this could mean more invasive massaging for patients.
About the same
I use all three incisions to place an implant - inframammary fold, around the nipple and through the armpit. I don't think any recovery is longer than the others if the dissection is done properly and carefully with an endoscope. I always use an endoscope with my armpit incision so that I can see every muscle fiber being divided versus some who perform the whole surgery bluntly. In those cases, I would imagine the recovery taking longer because of the blunt trauma.