Six months is definitely not long enough of a break for skills to degrade. But it's not just skills, as I wrote in another comment, the biggest thing is knowledge of new tools, new versions of language and its features.
I'd say there's at most around 2 years of knowledge runtime (maybe with all this AI stuff this is even shorter). After that period if you don't keep your knowledge up to date it fairly quickly becomes obsolete.
I would imagine there is probably some reverse S-curve of skill loss going on. The first year you may retain like 90% (and the 10% are obscure words, rare grammar structures, expressions, etc.), then in the next 2 years you loose more and more every year, and by the 3rd year you’ve lost like 50% of the language, including some common words, useful grammar structures, but retain common greetings, basic structures, etc. and then after like year 5 the regression starts to slow down and by year 10 you may still know 20%, but it is the most basic stuff, and you won‘t be able to use the language in any meaningful way.
I'd say there's at most around 2 years of knowledge runtime (maybe with all this AI stuff this is even shorter). After that period if you don't keep your knowledge up to date it fairly quickly becomes obsolete.