How long should I wait before planting soft fruit bushes in the same bed where old roses were?

No great reason to wait. After 20 years of roses the ground is probably impoverished so you will need to dig in some manure, compost or similar humus rich material. Then I suppose that October/November are the better times to be planting fruit bushes.

The simple answer to this is to scatter Rootgrow (Michorrizal fungi) into the planting hole when planting. This will alleviate any potential problems regarding replant sickness after the roses have been in the same ground for so long. This will especially be useful to the Raspberries and Blackberries as they are in the same plant family as the rose. Don't use it on the Blueberry, being and Ericaceous plant, it has it's own colony of michorrizal fungi species already on it's own roots and wont take too kindly to being invaded by other species.

Find more info here (copy and paste into your browser):

http://www.friendlyfungi.co.uk/rootgrow_information.php

Incidentally, David Austin roses do their own version and it's about 50p cheaper so instead of paying say £3.99 per sachet (goes quite a long way), theirs costs £3.49.