Most brewers on here will say that a secondary is unnecessary unless adding fruit or dryhopping in most cases. I plan on skipping the secondary when I resume brewing, as well.
By leaving it in the primary 3-4 weeks, it ensures that the fermentation is complete, plus it gives the yeast a chance to clean up after themselves.
You can always try guessing, and leaving it in the primary (or secondary if you decide to go that route). That's what I did for my first several batches until I broke down and got a hydrometer. But a gravity reading is the way to be completely sure.
Congrats on your first batch. I'm sure you'll enjoy the fruits of your labor, and embrace the obsession.