The Machinekanone (MK) 108 was developed by Rheinmetall-Borsig between 1941/42. The design was finalized in 1942. It met the RLM requirement for an aircraft cannon that could knock down enemy bombers with the lowest expenditure of ammunition and stay beyond the range of enemy defensive fire.
When did the MK 108 reach the eastern front?
Well the demand for these cannon were from those aircraft tasked with destroying bombers. I would imagine that most aircraft (day and night fighters) with mk108s were sent to the west front / defense of the Reich.
Messerschmitt stopped production of the 110 series in 1943. Production did continue at the Gothaer Waggonfabrik plant. With the availability of the DB605B engine, a nother version of the Bf 110 (Ausführung G) went into production, whose main features was the use of the DB605 engine and MG 151/20 cannons replacing the aged MG-FF cannons.
It had to be well into '43 before the 110G-2/R3 variants were produced.
The majority of LW fighters didn't carry the mk 108. That includes the G6, G14 and G10.
For example, the G-14 was (as the other 109s) produced by Messerschmitt in Regensburg, Erla Maschinenwerke in Leipzig and WNF (Wiener Neustädter Flugzeugwerke).
The minority was built by WNF. Many (not all) G-14s built by WNF had their MG 151/20 replaced by a MK 108, which resulted in the designation G-14/U4.
So the majority built by Messerschmitt and Erla kept their MG 151/20.
I don't have specific dates and numbers but an earliest best guess for 110G-2/R3 arriving in the east front would be late '43, most likely much later.
