I have mild ptosis and my doctor prescribed Phenylephrine Hydrochloride Ophthalmic Solution, 2.5%.

  This is a matter of opinion.  Personally, for reasons stated by others, I never prescribe topical phenylephrine for home use for ptosis treatment.  Ptosis repair is highly successful, not particularly invasive, treatment for your problem.

There is no roll for the routine use of phenylephrine in the management of ptosis.  It is strictly used for testing purposes.  Your prescription is out of the norm and represents off-label use of this product.  Phenylephrine will dilate the pupil which will affect your vision and increase light sensitivity.  It can also be associated with systemic side effects.  For this reason, we do not prescribe phenylephrine for ptosis.  If you have ptosis that responds to phenylephirine, surgery is recommended.  I encourage to seek a second opinion from a fellowship trained oculoplastic surgeon.  The American Society for Ophthalmic Plastic and Reconstructive Surgery maintains a regional directory on their website that can help you find a highly qualified ptosis surgeon.

Phenylephrine drops are safe for continued use as long as you are not at risk for "closed angle" glaucoma. 

Phenylephrine is routinely used for eye dilations and sometimes for ptosis testing to see if a droopy eyelid will rise after its application.  Unfortunately it can cause your eye to dilate and cause some blurriness/focusing problems.  As you found the effect is also very short.  A more commonly used option is to use brimonidine, which can help for 6 hours or so and doesn't usually cause any significant eye dilation or vision problems.  Brimonidine is a commonly used glaucoma medication and though it has some side-effect risks it is usually well tolerated 2-3 times daily.  Best regards.