help!

So I've just joined glow and it's so helpful! I've got two children 6/8 to past relationships. 
Fiance and I are TTC. I'm getting really upset. I mean this with absolute no intention of hurt as I know people try for YEARS. I got the mirena out 9 days ago, we've had sex every couple of days since then. Not had my period in 5 years. 
My question is when would I do a test? I have no idea when my period will come. I feel like a failure. I've had two beautiful children to two horrible fathers. My fiancé is an amazing step dad and I want to give him a baby he so desperately wants. I can't give him answers though as I honestly am so uneducated about fertility etc. Please no nasty comments as I'm genuinely just asking advice x 

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