Congratulations Hayate on starting your journey.
There is no single correct way to learn, so as you go, adapt to what works for you.
I do the lessons in the order they are presented (for the reasons Robert mentioned). The reinforcement builds skill and when you get to the toughest part (at least for me), the “Know It!” section, you have drilled the concepts several times. Now that I am on Level 1 Module 5, those long, seemingly impossible sentences are now flowing off my tongue. Keep it up, you will get there.
When I get to the end of a module, I go through each lesson in the module again. This reinforces the lessons I learned.
I also do daily Anki flash cards (more on that below) to help with vocabulary and learning the grammar (by speaking the sentences).
The one question I have in my mind is when I am going to do the Survival Kit section. I have decided to do that section at the end of Level 1, but I think doing one between each module (there are 10 of them) would also be reasonable.
As for how long, a little every day is better than longer a few times a week (or month!), so I set a goal of 300 points every single day (223 days is my current streak). 300 is not terribly difficult (it gets you through a lesson in 3 days typically) but it is an achievable daily goal. If I am feeling it today, then I do more, but I try not to do more than 30 minutes (I don't want to burn out). I also put all the vocabulary and sentences in Anki (a free flashcard program) and go through 20 new cards and the assigned review cards every day. I find Anki to be much more useful because they keep bringing up older stuff (and cards I had trouble with show up more often) and it is much quicker to flip those cards than it is to flip through previous lesson cards in Rocket Spanish. It is a pain to enter in all those cards, but I plan to make the cards public very soon (maybe this question will nudge me to do it today!). It is a program you use on your computer (the cards are stored on the AnkiWeb.net cloud). If you want a phone version, you have to pay $20 (as I recall). The phone version isn't quite as good as the PC version , but definitely works when I am out of town(I guess it is very similiar to the Rocket Spanish mobile app).
Good luck and enjoy!
Scott