How much he is going to serve in prison for his crime?

The film 'The Machinist' was set in California. Given that another person was killed in the accident (a child, no less), and that Reznik is intending to make a full confession (e.g. that he was distracted by reaching for his cigarette lighter) it's reasonably likely that he would be facing a felony charge for the accident:

...the prosecutor may choose to try it as a misdemeanor or a felony ...usually depending on the facts of the case and the defendant's criminal history.

If felony hit and run is tried as a misdemeanor, the maximum penalties are a fine of between one thousand dollars ($1,000) and ten thousand dollars ($10,000), and/or up to one (1) year in county jail. If the accident resulted in a death or a permanent, serious injury, in most cases the defendant will need to serve at least ninety (90) days in jail.

But if it is prosecuted as a felony, felony hit and run carries penalties of a fine of between one thousand dollars ($1,000) and ten thousand dollars ($10,000)...plus sixteen (16) months to three (3) years in state prison. If someone (other than the defendant) was killed or suffered a permanent, serious injury in the accident, the state prison sentence rises to two (2) to four (4) years. - _{^{SHOUSE CALIFORNIA LAW GROUP}}

So the best guess is that with a full confession and a guilty plea, he'd probably be serving the lower end of that sentence (2 years). Assuming he then demonstrated "good conduct", he would get a discount on his sentence of around 1-200 days, meaning that he would be out of jail in around one and a half years.