Based on the type of offences committed by a person from United States seeking to enter Canada, the time period of admissibility will vary. For an attempted assualt, a person can potentially qualify for "Deemed Rehabilitation" after 10 years.
Offences such as intimation but not actual physical voilence like criminal threatening, menacing or terrorizing are considered as potential serious crime in Canada. The actual details of such offense is important in determining if your application to enter Canada will be allowed.
If a person has been arrested in the past for assault and awaiting trial, there is a good chance that they could be rejected by Canadian Border official because Canada regards pending criminal charges the same as an actual conviction.
If you have an actual assualt record while applying for entry to Canada, the only way to successfully enter, would be to obtain "Canada Temporary Resident Permit (TRP)" or undergo permanent criminal rehabilitation. Otherwise under Canadian law, even if after 10 years of deemed rehabilitation, you could be considered inadmissible due to your original offence conviction which involved "Physical harm to anyone".
The Canadian Border are absolutely strict when it concerns entry and any person with a criminal history involving voilence hoping to enter Canada, will obviously face harsh scrutiny by Canadian Border Agents.