I totally agree with KT up to the point where he says;
I usually just pull out the roots to make way for more crop. I don't bother with the side shoots.
I grow Chevalier and Marathon although I gave Belstar a go last year and quite liked it. Giving it another go again this year (some seed left) to see if it a consistant grower.
Generally; I find that my varieties don't all head at the same time which helps to extend the season.
I find Chevalier heads about a couple of weeks before Marathon meaning that as I finish with Chevalier Marathon is ready.
Then when I have finished Marathon the side shoots on Chevalier are ready then a couple of weeks later the side shoots on Marathon are ready.
So it is not uncommon for me to be picking calabrese for two months or more.
So as I see it it is down to variety selection.