The first thing to know is that we cannot dread split ends! So if you have super long hair that you haven’t gotten cut in a few years, there is a good chance that you will lose a fair bit of length since the wispy, broken split ends you have I will recommend you trim off since I can’t do anything with them. The alternative to that is to pull the ends in and round them off. If you have split ends you may lose up to 6″ (for those with super long, 24″+ hair). However, it is a good reason to get your hair a nice trim a couple months prior to getting your locks so that you are stunned by the amount of length you may lose.
Another thing that can lead to loss of length is choosing to round/blunt the ends of your dreadlocks. Assuming you DON’T have any split ends (see previous paragraph to learn more) you can expect to lose 1 – 3″ inches in length by choosing to pull your ends in/round them off.
And, finally, if you have particularly thin OR fine hair. (Thin refers to the density of hair on your head; Fine refers to the thickness of the individual strands of hair) you may lose a significant amount of length. The exact amount is not really predictable, but this occurs simply because it takes so much more of your hair to create a solid, uniform lock. The thicker your hair is, the less likely this is to be a contributing factor.
The good news? The average person doesn’t lose ANY LENGTH! If you have been maintaining your “regular” hair and getting it trimmed regularly, choose not to pull your ends in (leave them wispy and loose), and have slightly-thinner-than-average hair or thicker you shouldn’t see ANY noticeable loss of length. 🙂 Most people don’t notice a difference at all.