Activity and inactivity is often a very incoherent mess no matter what role-playing forum you go to, mostly because writers are by very nature like that; they frequently get caught up in writer's block (I should probably make a point that I don't have this problem; the reason my work is slow is because I have so many notes to organize first) and stop posting as a result of it. Sometimes they also get inspiration and get involved in a thread only to abandon it soon after; many role-players are whimsical and lack devotion, which isn't a positive trait to possess and I usually use the presence of this trait as one of my focuses when deciding if I'll accept someone into my role-play in the first place. Life also simply happens sometimes and people can't be around. So on that note; yes, it sadly [i]is[/i] normal.

Given the nature of your question it might be better for someone more familiar with how this site works to tell you, since I imagine it would differ from one place to another. What I will say is that chances are this is an entirely subjective concept, so no one on the site will be able to give you an answer that applies to everyone (so assume any replies you get will be subjective); you should discuss these matters with the people you plan to involve in a role-play before actually getting involved with the IC at all.

On a side note, you get my thumbs up for wanting to write down and use ideas as fast as you can before they leave your mind; I'm the type of person who is being flooded by so many thoughts and ideas that it's virtually impossible for my body to keep up with them. A team working together would not be able to sort my project's notes efficiently yet I do that work each day; this is why all my notes exist in the first place and I'm currently not able to just sit down and write my story. I spend most of my time writing down countless ideas; whether they be for my project or for a role-play I'm in (or will be in). Doesn't mean I'll get to use all those ideas since (as I just said) role-plays die all the time due to many role-players having no conviction.