Depends on the software itself. Generally 30 days is more than enough to test it, but if you're unsure whether to buy it or not, then a slightly longer trial period would be preferred. Being able to activate a trial license more than once(atleast twice) would be ideal, especially if you switch software/hardware.
I am interested in how everyone will use the trial period. Do you actively test your defenses with virus samples, etc.?
I attach great importance to whether it really affects everyday use. Specifically, UI preference and extreme slowdown.
I can usually make up my mind in under 14 days, as to none AV software I do feel imitations are the right thing even though I find limitations extremely annoying at times it is the thing that motivates me buying the software - Which I suppose is the purpose of making any product?