  Effort estimation models are a fundamental tool in software management, and
used as a forecast for resources, constraints and costs associated to software
development. For Free/Open Source Software (FOSS) projects, effort estimation
is especially complex: professional developers work alongside occasional,
volunteer developers, so the overall effort (in person-months) becomes
non-trivial to determine.
  The objective of this work it to develop a simple effort estimation model for
FOSS projects, based on the historic data of developers' effort. The model is
fed with direct developer feedback to ensure its accuracy.
  After extracting the personal development profiles of several thousands of
developers from 6 large FOSS projects, we asked them to fill in a questionnaire
to determine if they should be considered as full-time developers in the
project that they work in. Their feedback was used to fine-tune the value of an
effort threshold, above which developers can be considered as full-time.
  With the help of the over 1,000 questionnaires received, we were able to
determine, for every project in our sample, the threshold of commits that
separates full-time from non-full-time developers.%, and that minimizes the
type I and type II errors. We finally offer guidelines and a tool to apply our
model to FOSS projects that use a version control system.
