Viruses, when outside their hosts, will gradually get inactivated. Several environmental parameters like temperature, humidity, UV index, determine this rate of inactivation or decay. For SARS-CoV-2, this parameter is not yet clearly known. This tool, based on Prof. Jimenez's work, uses a relatively conservative estimate which results in an additional loss rate of 0.63 per hour. For an estimate based on prevalent temperature, humidity and UV index, you can check the following tool by DHS.
Cells of SARS-CoV-2 are contained within infective particles, exhaled by an infectious person. However, these particles will, gradually, settle down on floor/ground. This settling of particles thus leads to a further loss rate which we have approximated at 0.3 per hour. For an example of settling rate of differently sized particles indoors, you may have a look at this work.