According to the goddess of poultry keeping, Victoria Roberts, a Wyandotte will lay approx 200 tinted eggs p.a. which will drop off as they get older. If you can find it, her book 'Poultry for Anyone' is worth a look. It has some useful comparison tables in the back and some fab photos of posh birds in the front!
Mine lay very well, but go broody every 10-12 eggs! So not very reliable. Very nice chickens, though. If anyone wants any I have gone a bit hatching mad!