Dose (J/m2) = irradiance (W/m2) x time (sec)

The irradiance will depend on measurement distance from the UVA source.

This is a tricky one to answer; dose itself is complicated, but irradiation from a source can be readily measured - what specifically is your source, and what is your application?

The integral of irradiance over time is  called radiant exposure, it can be assumed as a "dose" if a limit value is known.

For the protection of workers, for example, ICNIRP reports a limit on the radiant exposure that a worker may absorb in an 8 h working day, of 10000 J/m2.

Dividing this value for the measured irradiance, if everyting is constant in time allows to find a time interval that when exceeded causes the worker to be overexposed.

Notice that all  this is true provided than the limit for workers is of any meaning for your application and that irradiance has been integrated over the spectrum between 315 and 400 nm, see for reference www.icnirp.org and search for incoherent sources guidelines where definitions of quantities such irradiance, radiant exposure and so on are reported as well

For other applications, for example the detection of surface defects on steel products, another range is prescribed and the required quantities are reported in applicable ISO standards