An even older engineer once told me: Even after you've doubled the estimate, it will still take twice as long as you estimate.
"Six to eight weeks" was the default estimate my project managers gave for anything above trivial. Long enough to make the task seem difficult, not too long to scare off the client.
Ha!
This was the norm at my corp over 10 years ago. Heh.
Double it with each layer. Double your estimate for each individual task, then when you've got the whole Gantt chart built for the iteration, double that.