I'm a retired SMSgt. I made SSgt and MSgt on the first try. TSgt took four tries, SMSgt took six. Tech is a bear. Senior is a really really really big and mean bear.
It varies from career field to career field. Back in the early 2000s, they made a huge mistake in a few fields and made the promotion to SSgt super easy, so tons of SRa were now made SSgts and had nobody to supervise. It was done (we suspected) as a retention measure.
But ultimately they got screwed because with SO MANY SSgts, the cutoff for TSgt was astronomical. Some of those folks were 10+ years in TIS before they could get a TSgt line number. One guy was 14 years in before he made Tech. Pretty sure he was just hoping to get MSgt by retirement.
SMSgt and CMSgt are always tough stripes because by law, only 2% of the enlisted force can be SMSgt, and 1% can be CMSgt. Also, SMSgt and CMSgt are board reviews, you can't just test your way to rank. So if you have firewall five EPRs, you can basically test your way to MSGt.
But even firewall 5s and good test scores won't get you SMSgt or CMSgt.