I remember when I first started sentences, I had similar questions and whenever I saw people attempting to answer them, they were always very vague. I realized there was probably a reason for that. After 4000 sentences I know exactly why. We only add one sentence at a time, right? How much of your own growth can you realize when you're learning things one by one? This is why I can't easily trace back how I was doing at sentence 100, sentence 250, sentence 100, sentence 200. But I know exactly how it feels to want to look into the future and know how how 上手 you may possibly be at a certain point in sentence count.
Although I said all that, it doesn't mean I won't try to give you an estimate.
100 - At around this point I was just starting to speed up with adding sentences. My reading speed was still kind of slow.
250 - By around here I became more selective with sentence picking and definitely becoming more comfortable with it.
500 - By here I remember noticing some major reading speed increases. When seeing simple, commonly words like "時間," it took no thinking to recognize the word.
1000 - By here I knew I definitely had the hang of it. For example, words like: 間違える、帰る、変わる <ーーI began to sense that it was 間違える instead of the incorrect 間違る、帰る is more conventional than 帰える、変わる instead of 変る。I didn't think I would get the hang of this so quickly. At around 1500 I felt I was speed-reading through SRS content.
2000 - By this time or perhaps earlier I could automatically come up with the readings for kanji when I saw them. When seeing 低, for example, my brain could register the "てい” reading like that. This is all thanks for the SRS.
4000 - Just recently I noticed an increase in "comfort" while listening to Japanese. I can understand many things just listening passively. At the 3000 point and before, while listening I'd have to pause here and there to think about what was said if I was really curious.
And I feel like I can zoom through even non-SRS content, given they are all words I know.
(...that was actually easier to explain than I thought)
However, many other factors play into this. I estimated my listening hours are in the 5500-6000 range. Reading hours aside from the SRS are hard to calculate but I felt like I've done way more reading than SRSing. And also, my language exchange partner taught me a lot as well.
You will only know how it works out for you when you do it yourself! You'll probably see better results

やってみなきゃわかない!
Edited: 2009-04-27, 1:41 am