Denver Heat Question: When does my landlord have to turn on heat?

[Mindy Jensen]

Asking for a coworker. When does the landlord have to turn on the heat to the building? It's been high 40s, low 50s in the am here but warms up in the afternoon. 

3 requests to landlord have gotten the answer of end of next week.

Is there a temperature or timeframe the landlord must comply by?