How often are network/AC card drivers updated by the manufacturer? Where can I download it?

Depends on your PC actually and its system. You`d better should have a look at these Network card drivers https://driverpack.io/en/devices/lan  I trust the most. You see, if you need some reliable resource for drivers, then DriverPack is your option. The thing is that it will automatically select and install the required drivers

Broadcom rattle through the version numbers like nobody's business, with frequent new versions. This is not a good thing, but indicates how, in my opinion, broken their NICs and drivers are.

At the other extreme are Marvel, where updates reaching end users are rare. Again broken and staying broken. Often found rebranded for example by Toshiba.

Intel's updated drivers appear a couple of times a year. The exception that proves the rule is they left a SNMP management resources leak in their server drivers for over a year, as otherwise their rock solid.

There is a lot of variation in network cards, and brand quality changes over time. One piece of advice, don't rescue old NICs out of the WEEE recycling, they were likely put there for a reason.