I got the boarder patrol to add pedestrian wait times to their web site which is update hourly (on the hour), scroll down to bottom (San Ysidro):

http://apps.cbp.gov/bwt/

Heading south from Trolley across bridge to taxi stand takes less than 5 minutes.  I've never seen a delay heading south.  Heading north is another matter ...

Let's take these one by one.  And add some things you didn't ask about, but should know.

There's never a wait to cross into Tijuana.  Nor do you walk across a bridge to enter Tijuana.  

It would help to know where in Tijuana you want to go.  You cross a bridge to go downtown and to the Zona Norte.  15-20 minutes will get you to both places on foot.

However, I don't recommend walking across the bridge to the ZN/downtown when it is dark.  (In February it's dark at 6:30 pm.) Take a cab.  Saving $5.00 isn't worth the risk of being mugged, and sometimes guys get mugged on the bridge when it's dark.

Expect a long line at 8:30 am on weekdays to re-enter the US.  That's during the period when people who live in TJ and work in San Diego cross.  Plan on an hour, give or take.  (From 7pm to 4am the weekday pedestrian line is usually about 15 minutes or less.)

You can get Vitamin V in almost every pharmacy in Tijuana.  If you make a right turn just after crossing into TJ you'll see several pharmacies.  

The thing is, prices are highest in the areas most frequented by tourists.  That's the pharmacies at the border border and on Ave. Revolucion.

Better prices are found on Ave. Constitucion.  Discount Pharmacy at 6th and Constitucion has decent prices.  Of course, it you go to Otay, La Mesa, or other places tourist don't frequent you'll get even better prices.

You'll have no trouble finding tons of pharmacies before 8pm.  There are 24-hour pharmacies available if you arrive late at night; cab drivers should know them.  (I use  Gushers on Agua Caliente when I need a 24-hour pharmacy.)

I use Levitra, so I'm not the one to give you prices on V.  (And I have a swap meet connection for Levitra so I can't even give you the current pharmacy price.  What I can tell you is that I pay $6.00 a pill.) I haven't bought anything at a border pharmacy in so long I can't give you current prices on anything they sell.  

Going out on a limb, you'll save 20%+ going to pharmacy on Ave. Constitucion over buying at a border pharmacy.  (If you're just going to buy one V at say $15.00 shopping around isn't worth it.  But if you turn into a ZN "vet" the savings add up.)

The bridge you're talking about crosses I-5. A person does not necessarily cross it to enter Tijuana.  (For example, if you park at the UTEP parking lot you don't cross this bridge to enter Mexico.)

thank you WBs and Burt for the great info!