Ahhhh don't watch combo videos to learn stuff! That's like watching a Kung Fu movie to learn how to fight. Watch match videos of good players if anything.
Against comps, you want to tend to avoid those, as they will no doubt develop bad habits. In early stages of development, yeah you might want to get some tech practice in. Try to develop a solid technical game to have a foundation to build on.
Are you a complete starter or can you perform the basic techs easily? If you can perform them, great, now comes the hard part: using them properly. Play against humans, and try to apply your technical skills against them. Who cares if you get *****, in fact, that's a good thing, because that's telling you that you're using your technical ability the wrong way. Try varying it up (the exception is L-cancelling, you should ALWAYS be doing that no matter what). Waveshine twice instead of once and attacking. Try fast-falling all your ariel attacks. Waveland more (or less). Pulse walk more or less. Do what works.
The more you read, the more you know. I'm not saying you should go into char specifics and memorize every guide out there, but try to get a general idea of how playstyles work with different characters, how priority, hitboxes, and spacing works, and how you can use them to your advantage (attacking while landing behind an opponent for example). Learn what attacks to use, when.
This is where match videos come in. If your opponent is still beating you, try watching a pro vid of that very matchup, in which your character wins. I could never beat Space Animals with my Falcon, so I watched some Darkrain videos on how to do it. It was mostly heavy tech chasing to a finisher, and he started most of it by using very effective movement strategies and grabs. So now I need to work on tech chasing and movement and evasion.
There is no set time limit as to how much you should practice. You practice until you're not having fun anymore, because then you're simply not trying and it's not benefitting you.
