When to stop harvesting Asparagus, Rhubarb?

[Vanisle_BC]

How can I tell when it's time to stop taking asparagus & rhubarb stalks from the plants? Advice like 'stop harvesting in late May' & so on, doesn't make much sense because seasons vary and things mature earlier/later in different years. I've read 'Only take one third of the rhubarb stalks at a time' - ??? So next day would I take a third of what's left? Obviously not but what's a reasonable way to harvest, and how do I tell when it's time to stop?

How do other forum folks deal with these things?

[applestar]

Asparagus — I follow stop when pencil thin as a rule. When I tried stopping by calendar, they produced a lot more fat, edible looking stalks that I ended up letting go.