I'm not currently pregnant but have BF while pregnant twice. I would say don't make any plans for how long you are going to feed, just see how it goes, some people love it, some get the heebie jebbies. There's a LLL book about tandem feeding which is suppose to be useful (I've not read it, mainly because I didn't really think of myself as a tandem feeder when pregant with DD2), kellymom also has a lot of information.
I had just 18 months between my DDs so obviously was feeding very regularly during early pregnancy. I found it very hard to express milk from early on in that pregnancy but DD1 didn't complain about a drop in supply. She was an enthusiastic eater of solid though which probably helped, I have a friend who only has a year between her DC2&3 and she had to supplement with formula thanks to her pregnancy. I BF DD1 until I was 6 months pregnant and loved it the whole time, she just refused to BF a few nights in a row so I offered her some cows milk and that was it, very stressfree weaning, although younger than I'd have liked. She didn't show any interest in BFing when her sister was born good thing because that would have freaked out my mother
DD2 was a complete boob monster, but had a great latch and was a very efficient feeder from birth so it was a joy to feed her. She was 31 months when I became pregnant with DS and only feeding a few times a week which was not enough to maintain my supply and so she weaned when I was 3 months pregnant. At that age I had a running commentary on how much milk I was producing, first one boob stopped (the least favoured) and then the other, she still wanted to BF but it was painful for me so I said no after a couple of weeks. By the time her brother arrived she couldn't remember having milk which was sad.