The ideal harness line length depends on your height and reach, your riding style and the type of harness that you opt to use.
With the hook of the waist harness sitting a good 10 cm higher than that of the seat harness you will probably need shorter lines. For most people 24" lines are perfect for the waist harness and 26" if you like a little more space between you and the rig. If you are riding a seat harness then you'll definitely want longer lines. 26" tends to be the minimum for the seat harness with people opting for up to 32" or more.
The other reason that seat harnessed riders choose longer lines is riding style. Seat harnesses allow you a direct transferral of energy from the sail, through your hips to the board making it the harness of choice for most speed orientated sailors. To max out your speed you need to keep your rig vertical and your weight lower down. You need longer lines to pull this off. The waist harness, on the other hand, is a manoeuvre orientated harness. Having your weight a bit more up over the board gives you are bit more freedom of movement so shorter lines are a plus.
If you are not really sure what length to go with or you sail with both waist and seat harnesses then adjustable harness lines are a great way to go. They allow you adjust the length, enabling you to experiment and figure out what works for you. Just make sure that both the lines are the same length or you won't know whats up.