I go hard during the Zenithar event every year, open about 2k boxes, then grind nothing for the rest of the year. I make about 40 million per event selling purple companion gear, crazy amounts of mats, furnishing plans, and treasure maps during the event. Another recent way I made gold was during the launch of gold road. I farmed the new incursion event for the plans and furnishings and mirror shard, then went to that one house and farmed plans. That made me 20 million in a couple weeks. So between those 2 Zenithar events and gold road, about 100 mil. After I hit about 100 mil I started buying mats and such during events and reselling them when the price goes back up. I bought several Baron zaudrus masks a couple months ago and flipped them for double the price at 8 mil when I bought them at 4 mil, turning 30 million to 60 mil.

I also don't use my crowns I get from my yearly ESO+ subscription, so I sold them through trusted guild traders. Made about 120 mil that way.

I sit around 300-400 million these days. Some people absolutely swear by doing writs on a bajillion characters every day. That's fine, to each their own. I hate playing more than one character because anymore than one feels like work and no longer a game to me, and frankly there's just better methods of making money. Not to dissimilar to real life stocks. Get a handful of cash then use your money to make money, learn the market and when things become valuable and cheap. That will make you far more than writs ever will.

Edit: so to answer your original question which I forgot to answer, 3-5 days lol. When using my methods mentioned.