I used it before, but I prefer Jazzings and Beautiful Collections (I just do, don't ask why). You can do it the same day of relaxer without heat (cuticles are already open), and make sure you follow up with a good moisturizing DC. Your hair will be hard after you put the color, and after your DC it'll be soft again. Also, try to put vaseline on your hair line so if it drips your forehead doesn't look like stained Koolaid. And wear gloves, your fingernails will thank you. Rinse it all out until the water runs clear or your pillow cases and shirt collars will stain.
I think for me it lasted about 4 weeks- it fades with each wash and if you wash weekly that's like 4 washes. The bottle says 6 weeks, but honestly it's not as vibrant after a month for me anyway. I always do one shade darker than my real hair in a dark burgundy or chocolate brown.