How long?

How long does implantation bleeding last? Every where I've read has said 1-3 days.

What I thought was an early period (6 days early) has never gone beyond spotting. Specifically not even enough to fill 2 liners in a day and it's all been brown blood. It's been going on for 4.5 days now. Should I test now or is it too early to test? Or should I just assume it's a really light period and stop stressing? I was going to wait until it stopped to test but didn't know it was going to last this long.