I have had BPH for 20 years, taking Proscar, which has kept the lid on
things, but not yet ready to go for surgery.

Frequency has been my problem all these years (no problem
urinating)and, as you know, it can cause major inconvenience at times.

And definitely YES, there are periods when my frequency runs as close
as every 20 to 30 minutes.  And not just dribble, dribble, dribble,
but rather a full emptying of a bladder whose capacity is diminished
by an enlarged prostate pressing on it.

For me, this "rush hour" usually occurs sometime between 10 and 11
a.m. on many, though not all days.  It tapers off after about a couple
hours.  During that time I must have access to a rest room or be at
home.  Other hours, I find myself holding for at least an hour,
comfortably, sometimes two hours.

I consulted my urologist and also did some research of my own.  The
answer I've come up with is that these schedules are known as diuretic
rhythms.  Most men aren't aware of them because they have adequate
bladder capacity.  But, those of us with limited bladder capacity have
to empty more frequently and during those hours of high production, we
can't hold and must empty as often as every 20 to 30 minutes.

Don't take this as gospel.  But, it's one answer that seems logical
and which I've come to accept.  Would like to hear from others.