Both of my M3s have been sleeping for two (2) weeks, and it's going to be another 3 or 4 weeks before I can get them out on to the street due to re-paving.
I have been using the same BMW/Deltran Tender on both of my M3s.
Both M3s had about a 70% charge the first time I hooked-up the tender. It stayed SOLID RED overnight, then turned to the BLINKING GREEN in the morning, then it took another 6 or 7 hours to turn SOLID GREEN.
When I pulled the tender off for a couple of days and re-connected, it seem to take about the same amount of time Red until the morning, then blinking Green for over half the day, then sold Green.
The unit is designed to charge/maintain the battery at a SLOW rate. As a matter of fact, if you had a completely drained battery it will take something like 3 or 4 days to go solid green.
I hoped this helped...
2011 E90 M3 Alpine White M-DCT
2011 E92 M3 Individual Dakar 6MT