there are no units set in OGS but for very few constitutive relations which require temperature to be in Kelvin, for example. Otherwise the units are free for the user to decide. Most prefer to stay in SI system and then your end time of 20 would mean 20s.
For the second part of the question maybe @joergbuchwald can answer it in detail, but from lines for the RHS in THM FEM the dimensions for the weak integral are L^3/T, where L is the unit of length (say m) and T is the unit of time (say s). Then the source term integral must be of the same units. Then the values for the source terms are given in units of 1/T.
For changing the scales I would suggest to write down the weak form of the equations being solved and make a variable substitution for quantities you are interested. For example for time scale changing from seconds to hours all quantities related to time must be scaled with 3600 accordingly, e.g. the viscosity.
Not so easy for me to write down the whole procedure but the Wikipedia is a good starting point: Nondimensionalization - Wikipedia
I hope I could clarify some points.