First thing to note, Minecraft time-scale runs in increments called Ticks, a tick is 1/20 second.
These are all basic formulas if people are ever wondering. Ever.
UU Production Rate
It takes 10^6 (One Million) EU to create One UU-Matter.
During the process EU is taken and stored to generate UU and shown as a percentage.
U :: 1x UU Output shown in Ticks (Multiply result by 20 to convert to Seconds) E :: EU in Ticks U=10^6/E
How it works
It's just one million divided by the EU provided to the Mass Fabricator. It's just that easy.
Results will almost always be in decimal.
For an example. It takes 97 seconds to generate 1 UU when it is powered by 1 MFSU (512EU/t)
The packet sizes (ULV, LV, MV, HV) do not change anything, it's the EU/t that matters.