Kapish
Once you attempt your first theory exam, you have until the end of that month plus 18 months to pass the full set. Once you've got them all, you then have a further two years to have your PPL issued.
From CAP 804;
FCL.025 Theoretical knowledge examinations for the issue of licences
(a) Responsibilities of the applicant
(1) Applicants shall take the entire set of examinations for a specific licence or rating under the responsibility of one Member State.
(2) Applicants shall only take the examination when recommended by the approved training organisation (ATO) responsible for their training, once they have completed the appropriate elements of the training course of theoretical knowledge instruction to a satisfactory standard.
(3) The recommendation by an ATO shall be valid for 12 months. If the applicant has failed to attempt at least one theoretical knowledge examination paper within this period of validity, the need for further training shall be determined by the ATO, based on the needs of the applicant.
(b) Pass standards
(1) A pass in an examination paper will be awarded to an applicant achieving at least 75% of the marks allocated to that paper. There is no penalty marking.
(2) Unless otherwise determined in this Part, an applicant has successfully completed the required theoretical knowledge examination for the appropriate pilot licence or rating when he/she has passed all the required examination papers within a period of 18 months counted from the end of the calendar month when the applicant first attempted an examination.
(3) If an applicant has failed to pass one of the examination papers within 4 attempts, or has failed to pass all papers within either 6 sittings or the period mentioned in paragraph (2), he/she shall re-take the complete set of examination papers.
Before re-taking the examinations, the applicant shall undertake further training at an ATO. The extent and scope of the training needed shall be determined by the training organisation, based on the needs of the applicant.
(c) Validity period
(1) The successful completion of the theoretical knowledge examinations will be valid:
(i) for the issue of a private pilot licence....for a period of 24 months; the periods in (i) shall be counted from the day when the pilot successfully completes the theoretical knowledge examination, in accordance with (b)(2).
No time limit between exams/sittings.