Time spent won't be enough, and time spent doing things that you don't understand will lead to further reinforcing bad habits.
Have an idea about WHY you're doing each individual thing that you practice. Don't practice a combo unless you know not only how you're going to apply it in a match, but also something else you can do if they're defending against that particular starter. Taking Raiden as an example, you know his B2 and B3 lead to a high damage mixup in close, but do you have a plan to get in close? It's something that's built up over time of course, but you should always be thinking about how to add to that.
Adding to that you of course want to be looking at what your character can do vs the opponent's offense. Win or lose, if I'm playing online (rarely because lol online) and I see something that I'm not familiar with, I don't keep queuing up for more matches. I go straight to practice mode, explore that one string and maybe something that I think that character could mix it up with, see what it looks like on block and on hit, and come up with a simple plan that could deal with that scenario. Trying to explore all the options at once will drive you crazy, so it's something built up piece by piece over time, as long as you're taking the time to actually do so. Getting salty that you lost to some unknown thing and then going straight into the next match, especially when you're playing on your own online, isn't going to be as productive.
