I sent E-mail to Photon Light and they said:

Jeanette, as the whites (or any other two battery model) were not designed with a single battery in mind no testing has been done. That being said we'd guess you'd probably get around 25-30 hours of use from a single Cr2016 in a Photon 2. If you were to be using a Freedom model the variable light feature could extend this time even further.

Jeanette Isabelle
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I'm not sure whose twisted idea it was to put hundreds of adolescents in underfunded schools run by people whose dreams were crushed years ago, but I admire the sadism. -- Wednesday Adams, Wednesday