How many (rough) hours would it take for an 88 yr man (with 15% kidney function) to wake up after given fentanyl 25 mcg (for 3 times)?

Variable. Fentanyl is not excreted by the kidneys, but older patients can be pretty sensitive. It may take an hour or more, but really depends on individual pharmakinetics.
75 mcg of fentanyl. Is not a big dose for a normal sized adult (about 1mcg/kilo). I imagine that, unless it is combined with other meds, or the patient is otherwise debilitated, that this dose would sedate, but not knock out such a patient. It would take an hour or two to feel normal after this dose.
Maybe longer. I have seen the effects last upto three days in an older patient with kidney failure.