With no load a 12v battery usually has about 12.6v. The standard for acceptability is for the battery to maintain 9.6v during a load of three times the amp/hour rating for 15 seconds. This load is about the same as the starter if your battery is the right size.
So... ten volts after 15seconds is acceptable. Taking 15 seconds to start is unacceptable though.
Bosch Master, ASE Master, L1
33 years MB technician