How long does it really take to get meth out of your system, and what's the difference in time for it leaving if you are smoking vs. injecting it?

I see no distinction in research between smoking and injection. The common thought is smoking it leads to more toxicity than other methods, but there is no clinical evidence to support that.

The half life, (time at which half of the drug is still in your system), is upwards of 12 hours, however, if you are asking about detection of meth use in drug screenings, it can stay detectable in your urine up to 5 days, in your blood for over 24 hours and in your hair over 90 days. (All rates vary based on numerous variables).