"Reasonably good TSNR" depends on the scan parameters.
For 3 Tesla BOLD EPI data with 2-3 mm voxels, TR about 2 s, flip angle 50 degrees or more, echo time about 30 ms -- TSNR about 200 is common. But the real thing to check for is if some subjects have TSNR very different from the others. In our recent Shenzhen bootcamp, two guys from Chongqing had TSNR on 100+ subjects in the 180-210 range (from running afni_proc.py overnight, after they learned how on Tuesday), except one subject had TSNR=90. There was a serious problem with that data, and they needed to throw it out of their study.