We see this question a lot on internet forums. Although any dreadlocks that we start (or maintain) in our studio can safely and successfully have extensions attached to them immediately, the same is not necessarily true for DIY dreads that you have put in yourself (or had a friend put in) at home.
First of all, it bears mentioning, that in an ideal world, dreadlocks extensions should ALWAYS be installed WITHOUT the use of glues, adhesives, or string of any kind. These extra things can create a myriad of issues down the road including an increased risk for mold and can interfere with the locking process. (For more on how to attach dreadlock extensions, check out this FAQ post: How do you attach permanent dreadlock extensions?)
Because any and all extension installation methods rely on the existing dreadlock(s) an an anchor for the extensions, if they are not tightly locked enough, the extensions will not have a secure enough base to hold onto and they will just fall right back out again. (Possibly undoing some of the locking your existing, natural dreadlocks had done in the process.)
Your dreadlocks should be completely locked before attaching extensions. (See: What’s the difference between dreadlocks that are “locked” and dreadlocks that are “mature”? for more info!) They should be completely matted and have a solidly locked “core” to them. They should be minimally “spongey”. For some folks that can be as quick as 6 months. For those whose locks are started more loosely, may be treated with the wrong products/shampoos, or with neglect/freeform, can take much longer; 2 years or more. As you can see, it can vary pretty widely from person to person.
However, using the above link about locked, vs. mature dreadlocks should enable you to determine when you should safely be able to attach dreadlock extensions to you locks!