Elite troops constantly train. That's part of what makes them elite. Seals for instance after BUDS and SQT deploy into a training/deployment cycle that is roughly 6 months of training and 6 months of active deployment. An entire training and deployment cycle with qualification training is about 18-24 months.
As regular infantry in the Marines, we would do "maintenance" training when not part of a deployment cycle. For a 6 month deployment we would begin training 9-12 months prior to deployment, with the intensity of training ramping up as we approached qualification dates. I deployed 2 times for 6 months each (1 year) and spent 18 months (1.5 years) in training for those deployments.
That's over half my enlistment, and doesn't include things like jungle warfare trainining, combined arms training, arctic training, land warfare exercises, and the myriad other small unit training and individual skill training that was a constant. Having a day or two in garrison with nothing to do but Physical Training and classes/weapon maintenance was rare unless it was scheduled for higher command inspections.
The operational intensity of a unit such as Xenonauts would require a day or two of down time after a mission for debrief and equipment maintenance, then back to the shooting range, CQB rooms, classroom training, physical training, with maybe 1 day out of 2 weeks for "downtime". Assuming that you don't have an assload of casualties, of course.
Theoretically there would be minimum 2-3 training squads for every deployment squad in a rotating schedule (for each base). 1 squad would be the newest recruits adjusting to Xenonaut operations with a high emphasis on classroom and physical training, the next would be units that have undergone indoctrination and would empasize personal skills and unit tactics while studying mission debriefs to the last minutia, and the third would be a squad that had alien combat experiance. The deployment squad would be in a standby mode, maintaining equipment and practicing unit and individual skills with a lower intensity on class/physical training.
The first wave of Xenonauts would have been the best of the elite units from around the world. As the Xenonauts gained credibility and the world leaders recognized the need to fund and support Xenonauts, the non-xeno troops in various countries would begin stepping up training cycles for special forces and other elite units. They would have arrived at the first base at a minimum of a company strength unit, already educated and indoctrinated. The veteran Xeno troops would be assigned as small unit leaders and the training cycle would begin. There would be no wash outs because the troops arriving at the Xeno facilities have already passed the most difficult initiations. The only losses would be in combat or training injuries. Recruitment to non Xeno elite forces would be stepped up, even if some standards had to be lowered. Training cycles of elite units would begin to lengthen, and non elite troops would have their operational tempo increased to cover the gap created by special forces not being as readily available. There would be a very large pool (thousands) of elite level troops ready to deploy to the Xeno bases.
As least that's what would happen logically.