I have to see probation in a couple of days. I am so mad at myself because I messed up and did methanphetimes today. How long before I can test clean??

You more than likely will test dirty depending upon what type of drug testing they are using at where you are tested. What were you thinking? That you could just do this and it would not count? Be ready to face the consequences of your behavior.

What did you take, how much, how often, when is the test?

And, yes, I agree - there is no easy way and you will test dirty.

Approx 3 days in urine sample. Other than that your test will be clean after you've stuck to "quitting'

After amphetamine use, amphetamine but not methamphetamine is excreted
Into urine. Detection of amphetamines in urine typically can be for a period of several days after use,
Although there is reported detection for a week after prolonged abuse.

https://www.varianinc.com/media/sci/apps/dat05.pdf