According to Appendix E of The Lord of the Rings, ⟨ph⟩ stands for /f/
when final (because ⟨f⟩ is pronounced as /v/ in that position), and
otherwise is used instead of ⟨f⟩ either because it’s derived from ⟨p⟩
(in which case it’s presumably pronounced just like ⟨f⟩), or to
represent an especially long /f/. We can’t really tell which case we
have, but from the Omikhleia Sindarin dictionary [1], it appears that
all the long ⟨ph⟩’s are between two vowels, and all the short ones have
at least one adjacent consonant, so let’s use that as a rule and hope it
works out. (The Ambar Eldaron Quenya dictionary [2] is less easily
searchable, so I’m just hoping that this rule works reasonably well for
both languages.)

[1]: https://www.jrrvf.com/hisweloke/sindar/index.html
[2]: https://ambar-eldaron.com/telechargements/quenya-engl-A4.pdf