It's not always 300dpi

The ideal resolution (dpi) of your artwork depends mostly on how far away the artwork is from the viewer. Here's a chart to show you what resolution to aim for when creating your artwork.

Viewing Distance	Min Resolution
0.6m  /  2ft	300 dpi
1m  /  3.3ft	180 dpi
1.5m  /  5ft	120 dpi
2m  /  6.5ft	90 dpi
3m  /  10ft	60 dpi
5m  /  16ft	35 dpi
10m  /  33ft	18 dpi
15m  /  50ft	12 dpi
50m  /  160ft	4 dpi
60m  /  200ft	3 dpi
200m  /  650ft	1 dpi

The above resolution chart is based on someone with good eyesight and in good light.

A more detailed answer

I've written a whole article about choosing the correct DPI for print. So check that out if you want to get stuck into the details.

And if you want to see different example resolutions actually printed then check out The Print Handbook. Then you can see it for yourself.