I checked my 'Swift' last week, and they were ripe and ready to eat. Depending on when they were sown, and what part of the country you live in, home-grown sweetcorn should be ripening up nicely now. The tassels coming out from the top of the cob should look brown. Check cob development by peeling back the outer covering to reveal the cob. Just push your nail into the cob and a milky liquid should come out. If it looks very thin and watery then leave cobs longer to develop. However, if cobs have started to wrinkle up then you've delayed too long! Do not pick and store cobs if this can be avoided. Always pick cobs just before cooking. I'm told sugars in the cobs start turning to starch once they're picked, so freshly picked and cooked cobs will be the sweetest.