Let's put it this way - if you see a car parked outside with the glow of a laptop screen you don't recognize, call the cops. He will be there long enough for the cops to get there and sort him out.
For a simple analogy (numbers not actual, I have never actually tried):
1. If left open, it takes no time at all to "crack" into the wifi network.
2. If WEP is used, it could take as little as several hours.
3. If WPA is used, it could take several days.
4. If WPA2 is used, it could take several weeks.
Personally, since I have caught someone attempting to gain access to mine, I have take a few extra steps. I use WPA2 and I do not broadcast my SSID. It adds an extra step to the setup, but I know it is more secure.
If you are worried about yours, make sure you are running the latest firmware on your router. Change the admin password. Change the default SSID - I recommend using the hexadecimal equivalent to whatever word/phrase you want. There are plenty of text editors that will do the equation. Don't broadcast your SSID. Make your pass-phrase something clever and with more that one word. Use hex again, if you want. Most routers can limit the connections to specific MAC address and disallow all others. This step makes for more maintenance but no machine except those specifically allowed through their unique MAC address will be allowed to connect to your network.
Now, if you are looking to attempt it, be warned, you will get a ban here.