You can’t really have enough noise reduction ration (NRR), hearing loss can occur with continuous exposure to sounds under 85db so the more reduction the better.
I firmly believe that if you shoot a lot, you will suffer some degree of hearing loss, even if you religiously wear ear pro at all times.
Generally, I try to double up, wearing roll up foamies and electronic muffs. It depends on the activity and the duration.
Generally properly inserted earplugs provide the best protection, with the old foamie rollups going up to 31db NRR. Notice - properly fitted - http://www-esh.fnal.gov/CourseHandout_Mat/earplug.pdf
Muffs generally run from 18 to 26db NRR, with a couple that advertise up to 30. Fit is important on the muffs, also. They need to be sealed tight, a big problem is often the temples of the shooters eye-pro.
Double protecting doesn’t double the noise reduction. When using double protection, OSHA suggests adding 5 dB to the rating of the most protective device.
Fitting is important:
How do I use Noise Reduction Rating (NRR) value to determine the protection provided by a hearing protector?
NIOSH recommends using subject fit data based on ANSI S12.6-1997 [or most current edition] to estimate hearing protector noise attenuation.
If subject fit data are not available, NIOSH recommends derating hearing protectors by a factor that corresponds to the available real-world data. Specifically, NIOSH recommends that the labeled NRRs be derated as follows:
Earmuffs – Subtract 25% from the manufacturer’s labeled NRR
Formable earplugs – Subtract 50% from the manufacturer’s labeled NRR
All other earplugs – Subtract 70% from the manufacturers labeled NRR
TL:DR In other words, underestimate your protection and use as much as you can.