When do side effects start and stop with plan B one step?

Got my period on the 15 of November 2016 end on the 19 have a 26 days cycle that month had sex on the25 and 26 of november took emergency pill on the 26 took the second dose 27 around 10:30am now its the 4 of December and am having spotting could it be side effect are implantation bleeding took emergency pill before but first am experiencing spotting before my period

Plan B will take virtually immediate effect (otherwise it may not stop pregnancy!), & those effects can last up to a few weeks. For me, they always lasted 2 weeks. I would get very depressed with it, amongst other things.
You can expect any amount of hormonal symptoms, including breast tenderness, nausea, bloating, cramping, back ache, dizziness etc
Plan B is a large dose of synthetic (man-made) progesterone for your body to handle. Everyone reacts differently to it, & it tends to depend on where you are in your cycle when you take it, as to what symptoms you experience.
You may have spotting or bleeding after using it, & it may delay your period by a week or two.

Hope that helps.

I have taken it once and I never experienced side effects. Can you list which side effects in particular you are referring to?