I'm doing the same thing at the moment. Still on first career, but only March 1940. I'm betting five - optimistically. I'm going to play very defensively. I'm writing up every single patrol, so my character has a bit of a story and I actually want to keep him alive.

As a quick update

4 patrols (3 war patrols) in, I had quite the adventure, didn't even get to get out of the north sea before I had used up all my eels! :)

I was heading around England when I get a task force bearing down on me, already close to intercept, I scoot and go to scope depth, and I am sad to see it is just 4 escorts. they are moving fast and in a line, so i decide to attack, just as I fired eels no 4 one of them detects me, and all zag out of the path of the eels, and I find out I am only in 60 meters of water. Took some damage hitting the bottom in my crash dive as i didn't slow down fast enough. Spent 12 hours hugging the bottom before they gave up. I continued north.

Found myself a nice small convoy in the North sea, with 3 escorts, I do a textbook end around at flank speed, and sneak in and sink a Ore Carrier, a Mid Merchant, and a Pass/Cargo with a well aim's salvo of doom, my quick stern shot missed its target. I went deep, and they never found me.

I used the rest of my eels trying to pick off the escorts so I could shell the convoy to the bottom (Smooth seas) which failed, I even got a hit by the dang tribal class didn't go down.

So I should be close to 1940 now still on my first guy.