My friend's advisor in grad school (Physics, not CS) used to ask his students for various project estimates, and then he would double it and increase the units: 2 hours = 4 days; 1 day = 2 weeks; 2 weeks = 4 months; 2 to 3 months = 4 to 6 years = thesis project. My friend's estimated 3 month project turned into his 4 to 5 year thesis project. I mean, hey, it was experimental physics and his project ended up using a shipping container-sized faraday cage, scanning tunnelling microscopes, a clean room wearing a bunny suit, building stuff himself in the machine shop, writing software. All this for something that literally had not been done before and no one was sure it would work or what exactly would need to be done to get there (which starts to sound similar to some aspects of software projects). Plus the usual overhead like teaching ungrateful engineering undergrads (guilty!), hosting movie night at the lab and making liquid nitrogen ice cream, etc.