It's all depends on the plugin vendor and nopCommerce changes, from few hours to few days, weeks, months or never.
Most nopCommerce minor upgrades don't contain any or very few breaking changes, but once in a while, a plugin requires some major changes.
Yes Most of plugins are not upgraded to 4.20
I mostly recommend to use 4.2 version of nopCommerce. The main think is for this version i like the most is performance optimization.
for more detail you can check change list for this version from below link