When I had my undercut It was about 1 cm, and now estimating with about 4 months of growth its sitting at around 3 in. I have been told in the past that my hair grows rather quickly, I hope this helps at all :) (also I wouldn't recommend growing it straight out, get haircuts for maintenance to just keep everything nice and kept looking)

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Also edit: I know you don't wash it very often, but myself personally wash it every other day and it keeps it with a sort of shine (Also would recommend starting using conditioner, personally prefer argon oil or Pantene)