I have taken the CAE (Cambridge Certificate of Advanced English) which is one level below the CPE and I was told at the time that the CAE and CPE do not expire at all.
But check with the program you are applying to, because they may view this differently.
This is what Cambridge English Language Assessment has to say about the shelf life of their certificates, including CAE, CPE and IELTS (emphasis mine):
Shelf life of certificates
We are sometimes asked how long the Cambridge ESOL
certificates last, or whether a candidate who took an exam some
years ago needs to retake the exam.
The simple answer is that the certificates do not expire. They
show that on a particular date the holder demonstrated that they
had attained the specified level of language skills. For most
candidates, the certificate is the result of a specific preparation
course and serves as a mark of achievement in completing the
It is clear, however, that language skills can diminish over time –
a phenomenon often referred to as ‘language attrition’. In deciding
whether to rely on a certificate obtained some years ago,
educational institutions and employers need to take into account a
number of factors, most importantly whether the holder has kept
up his or her use of the language and whether the level of the
certificate is significantly higher than that required for the job or
course in question.
There are therefore no hard-and-fast guidelines for the period
since obtaining a Cambridge ESOL certificate after which
additional evidence of current language ability may be required by
employers or institutions.
The Test Report Form provided by IELTS is not a certificate since
it is not focussed on a particular level of language ability; for this
reason, the normal shelf life for an IELTS Test Report Form is two
years (see under Results in the IELTS Handbook).
My conclusion: CAE and CPE do not expire. IELTS doesn't really expire either but there is a 2 year recommendation.