How to know how long a casting director will watch your showreel for before making up their mind
Put yourself in their shoes. You have a busy day and a zillion actors at your disposal. You are looking for a particular fit - the right look, the right age & build, vocal strengths, and whatever x factor is required.
10 seconds is enough to judge 90% of this criteria.
So you make a quick decision about whether to devote your time and view longer or pass to the next candidate.
30 seconds is enough to assess the acting ability and judge if this might produce an appropriate x factor bonus that will please the director (do they work well with their eyes? Do they create emotion in the viewer? Do they have that extra spark that keeps people watching?......)
60 seconds, and you have seen enough - yes, put them on the short list - maybe, a back up if needed - no, didn't really get there this time.
I am not a casting director myself. I am an actor, music producer and presentation skills & media interviews coach. In these capacities I have experienced all types of decision makers - from 'cattle market' stage show auditions to TV script reading auditions for major roles. I have sat in front of record companies whilst they assessed my music. I have seen how much time a music producer devotes to listening to the hundreds if samples sent to them. I have worked in hundreds of corporates seeing how they go about their recruitment selection process. In every instance the same applies. The initial decisions are made in seconds. Once you are through the first round, then you get more of a hearing, but that first look at you might only be seconds.
So don't put together a showreel that builds up to your best performance after three minutes because it may never get seen.
My advice would be to have some very brief 10 second bursts depicting the best of your work to open your showreal followed by a couple of longer 30-40 second examples (which could include lengthier repeats of the opening bits). Again, don't save the best till last, mix them up.
Remember to hook them quickly and hook them regularly to keep them watching. The overall timing of your show real should more than a minute, but not more than three minutes.
I hope this helps
Good luck my friend!