[cloakandswagger]

I would estimate a reasonable amount of prep time prior to interviewing is around 100 hours. This is about how long it will take to brush up on and practice data structures, algorithms, DP, design questions, etc.

This is about 2 months of practicing an average of 2 hours a day, meaning you're still working a day job. I figure this could be reduced to 1 month of prep if you quit your job and have all day to yourself.

[gaylemcd]

Lots of people get offers with little to no prep.

[dnackoul]

Usually these are people who are coming from companies with similar interview styles and are passively prepped by being an interviewer (for algos/coding) and designing web-scale systems (for system design).