006 - When is an army completed?

I have been dry brushing Leadbelcher Metal onto Chaos Space Marines for over a year now and it needs to stop. I have about 2000ish points worth of models in my Iron Warriors army and I'm asking myself, when is it competed?
"Never!" I hear you cry.
But bare with me. Some people will only ever collect one force whilst some people will collect army after army. the multiple armies are more of a trend now that allies are firmly entrenched in 40k. Unbound armies have the potential for people to collect more than one army due to buying just one unit for a new force and then another till lo and behold you have the skeleton of new army.
However there are many obstacles to having multiple forces. The main constraints being cost and time.
This is a really expensive hobby and starting a  new army does cost alot. A small legal force can still easily cost the thick end of a £100 for a HQ and a couple of troops. This goes almost double for Forgeworld models! A solution to this are second hand armies that can be a bargain on sites like Ebay and can sometimes be painted and a decent size for a lot less than if you have collected it yourself.
I'm a really, really slow painter. So the issue of time is a big thing for me. I am so jealous of people who can knock out a great army in a few months while it takes me over a year to get a scrappy looking one. The time issue also stops any chance of getting an army which is the hot new thing because by the time it is done a newer shiny army has come out!
People can finish an army for different reasons. It can be the end of the project, for example an army for a tournament or event. Some people have a list and when they have every model in the list the army is done and they only go back when they the codex is changed or refreshed.
Speaking of Codexes if an armies rules are completely out of date and non-competitive such as Sisters of Battle players stopping using their army. This can also happen when a Codex is new but not that good such as the recent Tyranid Codex and the Chaos Space Marine book. There is also the issue of armies being "squatted". This refers to the Squat army that was written out of the universe and hasn't had rules since 1993!
People can get bored with their armies and get their groove back by starting a new one. This can be when they have started with an army that doesn't lend itself to the player's play style. For example someone trying to sit back and shoot with an Ork army when maybe they should have started with Tau.
The reason I want to get another army is that I want my painting to get to the next level so the way I have done my Iron Warriors is very simple and I want to challenge myself.
I can always go back to my Iron Warriors, besides that Khorne Deamonkin Codex is looking quite tempting. Having a few Deamons squirming through Iron Warrior plate is quite appealing!