Training just enough to maintain your level is the worst idea.
You don’t need to “train” to maintain your skills, using them will maintain them. If you don’t need particular skills for a longer time, they will gradually deteriorate. I rarely use all the math skills I have picked up, so I periodically do a fairly intense refresher on things I haven’t been using, then let it slide until it is needed, or I think I have forgotten enough to do another refresher.
You don’t need to “train” to maintain your skills, using them will maintain them. If you don’t need particular skills for a longer time, they will gradually deteriorate. I rarely use all the math skills I have picked up, so I periodically do a fairly intense refresher on things I haven’t been using, then let it slide until it is needed, or I think I have forgotten enough to do another refresher.