Maincrop length

Im sure everyone is fed up of potatoe questions on here, a corker though, how long do main crop stay in,mine are Cara and ive worked out theyve been in 17 weeks so far, they are starting to show signs of dying, yellowing etc and have flowered very heavly. am i right in thinking 18-20 weeks is they usual?

from Johns potato page:-
First Early, Second Early and Maincrop Potatoes
These terms often sound mysterious but they're not really. All the terms refer to is the time it takes from planting to getting a crop. First earlies are usually ready in around ten weeks, second earlies in around 13 weeks and maincrop after about 20 weeks.

The biggest yield will be after the foliage has died back, though

  DD, i will let them die back, thanks guys, i think commercial growers plant later and leave them out longer hence the October comment