When you are ready!
It's suggested that the applicant take multiple practice examinations online. https://hamstudy.org offers excellent resources for those trying to obtain or upgrade their FCC amateur radio license.
The PVVET feels that when the applicant starts averaging 80% or higher on the practice examinations, the applicant is ready for their examination. We also suggest that if possible, to look at the materials related to the next element as some of the questions from one element can also apply on the others.
If you are ready, you can look at our schedule for the next PVVET sponsored examinations. If not, you can also look at the Laurel VEC schedule. You can  also find an exam on Hamstudy.org or visit the ARRL's find an examination page.  FEEs may apply when using other VE teams/VECs