Beetroot normally take 7-8 weeks to be ready to harvest after planting out. It's better to harvest too early than too late as they can become woody if left too long. You could gently scrape away some soil from the base of the leaves above where the beetroot is developing to check the diameter of the beetroot. Alternatively, just pull one up to see if it's ready, the rest should be a similar size so you'll know whether to leave the longer or not. Harvest when the size of a tennis ball, or harvest smaller for baby beets.
When harvesting, loosen the soil around them and pull by hand. Remove all but half a cm of the leaves, and store in cool dark conditions or in moist sandy soil if not eating right away.
You can find a complete growing guide here: https://www.littlegreengrowers.ie/how-to-grow-beetroot
Want to sow more beetroot seeds? You can find them here!