I'm 23 weeks now and we'll have about the same gap as you by the sound of it. We haven't not talked about the baby in front of her IYSWIM but we haven't specifically told her. I feel the same as you and don't want to have to try to explain how long x months is!
We have started reading a book called Special Something where the mummy has a great big tummy! She really took to it and we read it about 500 times the first week but still doean't know there's a baby in my tummy but at least when we tell her she'll have some understanding. A couple of times she said there is one in my tummy but I just deflected with 'Mummy's lunch is in her tummy!'
I'm going to wait til after Christmas I think, but I'm still hardly showing and didn't get huge last time. If she points out the big tummy before then then we'll probably tell her. Also I don't think you need to try and explain a timescale - just say it's in there to start with and will be coming to live with us one day. I think they're unlikely to question it further.
Sorry that was quite long! But anyway, that's my experience so far. Hope you get some replies from people who have actually had the 2nd one!
I am in the same situation. dc1 will be 3 when dc2 is born. Getting her to understand what is happening won't be a problem. Getting her to wait patiently will be. Therefore I am hoping to leave it as late as possible before we tell her. I was hoping to leave it until just before we decide to start getting baby things ready. Mind you, at the rate I am growing we might need to tell her before then
We told DS1 early (about 12 weeks) and got him to tell the rest of the family. However, most people misheard him and thought he said he was getting a wheelbarrow! DS1 was 3 when DS2 was born. I found that I would have had to have told him by 4 months or so anyway because he is quite boisterous and charges around. He needed to be reminded of being careful of the baby in Mumy's tummy. He bonded with my bump and took to being a big brother like a duck to water.