"Easily adjust and change the parts", seems to address your concern about it. For the most part, templates are made to be individually customized.
As far changing duration, if you know AE, it's generally not overly difficult. The big problem, is that most template projects use lots and lots of precomps so that if you want to change element A, it ripples though the entire precomp. Same for Element B. So to change the duration of certain things, you need to change the duration of every single instance/precomp in the project. You probably should look at this free script, rd: Comp Setter, it allows you to select the final comp, make adjusts that ripple back to all nested comps.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.
There's a way to ask the Author of the template directly by leaving a comment or contacting them via their profile page. Most Authors are responsive to pre-sales questions (I know I am!). As it's been already said, you do need to be aware of the project's structure- ie pre-comps/nested comps when changing anything in the project.