If they are 1st earlies or salads when they flower is time to dig them up because you want them to be small. This is usually about 100 days after you have planted them.
For main crop its about 120-150 days. I have seen conflicting stories about pulling the flowers off and leaving them on (uses up energy).
They are said to be 'ready' when the bottom leaves die off.
Me, I just dig up a plant to see if they are ready.