How long do we have until our political beliefs make us ineligible for security clearances, reenlisting, even enlisting in the first place?

Won't happen. Political leanings aren't the problem; extremism is the problem. Two completely different things.

Depending on the political belief, this ALREADY happens. KKK members cannot join up. Nor can avowed racists. (Avowed being the operative word, there). Nor can people who are actively seeking to undermine or destroy the government.

I do not feel that our political leanings will make us ineligible for enlistment/reenlistment nor security clearances nor should it be used as such. Here is my thinking.....If we used the political leanings of folks as a basis for qualification, then there would be no one eligible to join. Look at it this way: If no one that registers as anything other than Democrat can enlist....how many folks in the Nation does that cut out for eligibility? Same thing for Republican? Or even Libertarian? If stats are true, then currently 1% of the Nations people meet the basic requirements to join. Of that 1%, only approximately 0.5% actually join (give or take). According to an article in ArmyTimes from 2017, the target audience for enlistment consisted of 33.4 million. Of that, only 9.7 million were qualified to join the Army. Of that number, it was figured that only 136,000 actually joined. The branches have enough of a hard time to meet target numbers. Throw in political beliefs and you've just killed your recruiting mission.