how can one estimate complexity of task at hand?

There is this whole Computer Sciences way of doing that, O-notation etc... but it is largely theoretical and concernes with algorithms, not tasks.

Task like 'create user' is numerically trivial, but may be extremely time consuming when you're talking to Active Directory.

How can I estimate if I'm at all able to accomplish something like 'Create Interface Module For Loudmouth Library'?

that would require using Inline::C or h2xs, following on that question, how do they compare in terms of required knowledge and free time?

UPDATE Loudmouth is C jabber library, it works where Net::XMPP doesn't. I can either work to fix Net::XMPP or expose loudmouth to perl.That is what triggered thoughttrain on the subject.

To clear things a bit - solving Travelling Salesmen Problem is trivial as a task, because there are easy to use libraries (like Graph::Kruskal) that solve this, and it's just a metter of massaging your data to feed them.