According to the source “Jagdgeschwader 51 “Mölders” By Aders and Held, Motorbuch Verlag, 1985, P.84-85:
JG 51 had received the new Bf 109F-2 before the invasion of Russia, except for 2(Jabo)./JG 51 who retained their E-4/N and E-7 for their ground attack role.
In the beginning of the year first the Stab, then I /JG 51 followed by IV/JG 51 got the F-2.
In May 1941 the remaining Staffeln got the new F-2, while the units were based in Krefeld, Düsseldorf and Dortmund, after being pulled out from France. The emptied airfields were left with aircraft decoys to disguise the withdrawal of the units in preparation of operation “Barbarossa”.
On p. 89 it is said that despite the effectiveness of the Bf 109 E in the ground attack role when compared with the F version, 2.(Jabo)/JG 51 had to turn in their Emils for the new F-2 after a few weeks!
I have not yet found info confirming the data compiled by Michael Holm on the site which Leo posted, that the Bf 109 E was used operationally by front line JG 51 units until early 1942..
The upper listing, Gruppe by Gruppe, does not include the "Emil" listed among the operational units. Perhaps the older aircraft were retained by the training units of JG 51. The release of the last Bf 109 Es listed under IV/JG 51 in May 1942 seems to coincide with the fact described as:
Disbanded 2.42:
Stab of Ergänzungsgruppe/JG51 became Stab/EJGr.Ost
1. Einsatzstaffel/JG51 became 12./JG1
2. Ausbildungsstaffel/JG51 became 1./EJGr.Ost
The Bf 109 E seems to be leaving JG 51 as per:
March and April 1942 for IV/JG 51
March for II/JG 51, except for a few E-7s received and but also left the unit during May and June 1942.
I/JG 51, which ought to have been the main user of the Bf 109 E , released the bulk of the remaining Bf 109 Es in April 1942.
I believe the timing can be seen in the light of the reorganization of the training units in February 1942.
The very last few "Emils" listed can of course have been kept as unit "hacks" in the late spring of 1942.
Pictures of 2./ErgGr JG 51 in Düsseldorf in early 1942 show the use of JG 51 marked Bf 109 Es, which can support that these older aircraft were actually used by the training units of JG 51 and not the front line units.
I believe someone else can add more on this subject.