When to pick tomatoes

This is quite normal - It's not often that all the tomatoes on one truss are ripe at the same time. The supermarket varieties are probably bred so they are more likely to do so, but even then you get part trusses in the boxes.
I think you'll find most people pick individual tomatoes as they ripen.
Ripeness depends on variety,  but if you wait too long they will be tasteless. the texture can also be mealy and they fall to the ground eventually.
Try tasting a few at various stages to see at which stage you prefer to eat them.

I had to pick all mine urgently this year because of blight but the green ones did eventually ripen off the vine, even though some took a few weeks. I just left them outside layed out on the patio. Generally I think its best to pick and use as they turn red so the plants effort is directed at those still to mature.