The beets and radishes will tell you when they're ready by looking at the tops of the roots where they join the greenery... You can actually see how big they aill be as the root pushes out of the ground. Remember, the radishes need lots of water, especially if it's oberly warm. From planting to harvest the radishes will take about 3 weeks. Don't forget to replant as you harvest them to assure a continuing summer long crop. The beets take quite a bit longer though... maybe 6 weeks depending on the weather. Don't throw away the tops... they make a good sald (the smaller ones) but are especially good when cooked gently... in a little water, just until the leaves wilt. Then cook up about 4 or 5 slices of hickory smoked bacon until it crumbles in the pan, add about 1 cup apple cider vinegar (carefull! don't let it spatter hot grease on you), add maybe a small palmful of sugar, stir until sugar dissolves, add a goodly dose of freshly ground pepper and pour over the greens. A real favorite here in the southern U.S.!
Can't help you on fennel since we only grow it as an herb, using the tops for flavorings... Enjoy!