I really admire anyone who can eat rhubarb just reading about it sets my teeth on edge.
Looks like it shouldn't be eaten from late May onwards. Found this on a website:
After late May to early June, it is time to stop harvesting and allow the plant to produce summer growth for continued bearing the following season. In some seasons, rhubarb will produce seedstalks. These should be cut and discarded immediately as rhubarb that produces seed also produces less foliage, resulting in less vigorous crop the next year.