By chance, I found a query that, I think, it can give us a rough answer:
Click on it and look at the line (on top of the page) that reads "Results 1 - 10 of about ...", it was 25,270,000,000 when I checked.
Let's guess how many time it would take to crawl all those pages?
If it was a second per page... that's about 421,166,666 minutes ...or 7,019,444 hours ...or 292,476 days ...or more than 800 years. Even if it was 10 pages per second... it would be 80 years!
Out of curiosity, let's guess the other way around? hmm... say they need to crawl each page... every 10 days? That's about 105,291,666 pages per hour ...or more than 29,000 pages per second.
How come they crawl our sites several times a day!? OMG!