I haven’t stumbled on that part of the Tofugu guide in a while, but I thought at least the old WK guide said to start grammar at level 10 “if you haven’t already.” So the implication being more to start it at 10 at the latest. I can easily be misremembering, though. ^^
With regards to your question: I took the free levels very slowly as I was still trying to understand the ins and outs of kanji, hadn’t committed to doing WK daily yet, and started with a very low number of lessons a day. All that made it take longer than three months for me.
And average hours per day? At the beginning, with the review load not being very high yet… between one and two hours a day of WK work, I’d say. In the beginning, I also remember being a lot slower with my reviews.
So while that time a day may seem much, it’s not like that bloated to 4 hours a day of just doing WK. Workload increases over time, but so does your ability to learn and review. Basically every skill can be honed and improved as long as you do it every day, learning included.
And while I think it’s always wise to be aware of the official advice - you know what you’re capable of. If someone is already quite familiar with Japanese, and knows some vocab and kanji already, there is no point in them keeping to the advice of delaying crucial parts of study. The only thing you have to ask yourself is if you’re over-doing it, and if you can keep up that workload for months or even years.
I personally feel like they advise people not to start all at once just to minimize the number of people that will start everything at the same time while being potentially unfamiliar with kanji / Japanese, and then get overwhelmed, frustrated and burnt out in a month.
Don’t be scared about putting together your own learning routine. You know what you have time for, you know if you’re having fun, and you know if you’re retaining and understanding the things that you learn. If the answer is yes to all that - you might be able to add more onto your routine. If the answer is no - you should likely look into (temporarily) altering your routine to keep it sustainable.
Best of luck!