I used to work in an organisation that set a limit of 20 days effort, below which a task would be allocated to a technical lead who would be told to "just get on with it". Above this size, the work would be allocated to a PM, and would be subject to project disciplines (albeit very lightly applied at the smaller end). Ignoring the question of who produced the initial estimate, how do other organisations decide whether to use a PM and manage small pieces of work as projects?
All temporary work, no matter the size, requires project management and the PM disciplines that go with it. The smaller and less complex simply require less formality, rigor, and documentation of those disciplines, but they still exist.
So whether you formally assign the role PM to a human, someone on that small team will assume those duties. Either they are legitimately assigned or they will assume it. Obviously, if the task is small, you will not burden it with a full-time PM. It would not require 40+ hours a week, but it will require some hours in that week. Either a full-time PM pops in and performs that work along with other projects or a technical lead allocates a portion of his time to it. Either works.
I would say a work may not need PM if it's isolated. I mean, if it's only one person doing it, and (s)he has not to insert it into a tight schedule, and it only needs resources available all the time, why bother for complicated planning and organisation? Just get it done.
Which means most of the tasks will need some sort of PM, of course...
That 20 day rule for 'just getting on with it' seems fine as long as the project is properly specified - as with many things if the specification and estimate are accepted then you are in a decent position.
I can't see where the PM will really be able to make a difference without significantly cutting into the project budget on that size of project - but I would like to have the technical lead regularly feed back process (perhaps in a daily 15-minute agile stand-up), otherwise it might still be 20 wasted days.
At my current employer (unless otherwise specified by federal or DoE regulations) tasks more than 3 weeks long have PM supervision.
At my previous employer, the project manager was responsible for allocating all work of any size. Basically, his title was "PM" but he was just a manager. The pointy haired boss hired him because he got tired of dealing with developers.
In our shop, everything smaller than 50 (ideal) days is treated as a request and (when approved by continuity steerco) processed by continuity teams. We have coordinators (they're same positions as PM's) who manage the throughput of these requests, so there's is no 'just get on with it'-mode. As from 50 days (and usually multiple involved parties) a PM takes over, with 'light' methodology. From around 100 days no excuses left: full PM Methodology to be followed.
There can be exceptions, but these are the general principles.
whenever the resource that in charge of doing the job is not capable of reporting and following up on missions. As project managers, we often don't contribute much to producing value (code, design, work process), but we are there just to make sure communication, order, work procedures and timelines are all in place. thank god we're there...
Based on previous XP : dedicated PM (ie specially appointed person, even if not working 100% on PM) when total workload above 50 days.
Now I'll answers as everyone above : - PM = 15-25% of total workload - Project management is needed all the time, depending on the size of the project it can be more or less formal and complexe. If no full time PM is required, either the PM can work on many projects at the same time, or one of the team members can be acting as PM (and have dedicated time for this).