1-2 months at most. 1 month if you're reading full-time.
I did IM in 20 days, but I was reading on and off due to family reasons. I could've done it faster. But it's still the biggest and most important book. I finished reading pediatrics, surgery, and OBGYN in 20 days. They didn't take much time as I expected (I read around 50 pages per day). As to psychiatry/epid/ethics, I found it's almost identical to behavioral sciences in Step 1, so I won't read it. May be I'll just search for new topics in it.