You use the word "spend" with an amount of time because we think of time as being very valuable, just like money. To "spend time ___ing" means to use your time to do it:

We spent the first month just debating what to call ourselves.

People use the phrase "spend (time) as ___" to express how long they did a certain job for. You use this phrase when you're describing your past jobs, but usually not to describe your current job.