go by feel, go by time. if you feel exhausted all the time and want to sleep. if your diet has gone to crap and you have lost significant weight. if you start laboring through runs, no top gears, and your times go backwards.
but this is also the middle of summer and heat and humidity can also knock you down. my suggestion is push your runs back earlier or later. i ran daybreak or after dark. do that, see if you bounce back or still feel like heck.
last point, but if you are preparing for a season or event, keep in mind the goal. all manner of nincompoops on here who seem to think the mileage is more the point than being healthy for their season, or preparing for and setting PRs at races. if you start to feel unwell or physically beat up, it's not even the season/event yet. ease up.
personally i think this is all overrated unless you are pro level. it reminds me of my college soccer coach who would run us in the ground preseason, get early results, but the team would be suffering attrition problems by october. the idea is to finish a season without breaking. the idea is to not peak meet or game 1. to me, this is prep work, not the season, if you feel off, back off or come at it different.