I believe it's five years from the time of the crime.
Here is sauce for my previous post:
"The statute of limitations for obstruction of justice usually starts from the date that the crime actually started. The exact statute of limitations will vary depending on the type of crime in which the obstruction took place. Usually, the statute is six years in state courts, and five years in federal courts, with the exception of crimes against children, sex offenses, certain violent crimes, and murder."
federalcharges.com/obstruction-of-justice-laws-charges/
Since President Trump committed no initial crime, there would seem to be a big problem with pursuing an obstruction of justice charge.