Hi StressTest.
To "maintain" what you have done so far, and make some progress in GMAT verbal, you could do things that will stick even though you aren't going full force on GMAT preparation.
So, you could:
-Study sentence correction concepts, such as modifier use and placement, parallel structure, and logic of comparisons. Once you get concepts like those down, you can apply them in your writing and speaking, and you will basically understand them from then on.
-Learn the various types of Critical Reasoning questions and how to go about answering questions of each type. Once you really get these concepts down, you should retain them pretty well.
-Work on Reading Comprehension by carefully going through one or two passages for your half hour or whatever time you would put in.
-For quant, you could work on one area of quant, such as an area that you are weak in, at a time, as you have time. Once again, if you really deep dive into a topic and totally get it, you should retain what you learn.
Meanwhile, much of what scoring high in verbal takes is seeing what there is to see in the questions and answer choices. You can always develop this skill by doing a few practice questions very slowly and carefully, analyzing each choice and articulating why each choice is correct or incorrect. So, working carefully on some practice questions as you have time over the next few months could be very productive.
Then, once you have time to work on GMAT prep more, you will be familiar with the type of thinking necessary for arriving at correct answers and primed to make good progress.
You don't necessarily have to work on the GMAT every day. You might be better off spending a solid hour or two on it whenever you have time, maybe one or two days a week or on one day on the weekend. The great thing about working in this way is that, over the next couple months, you could become more familiar with and comfortable with how to answer quant and verbal questions in a low stress way, as if prepping were a hobby. Then, once you do go all out, the GMAT will be your familiar game, and your full time prep should be super productive.