I got my first mythic I believe during my first two or three months of playing. It was Scorpius. I tell ya it’s a total crap shoot only because I hear there are people who played for over a year and not get one. I personally just accumulate resources for about a month and then just open them all up at once. I got an average of one mythic a month for like four months straight doing that. The other mythics I got were all just luck from opening up chests at different times.
My suggestion is this: Don’t worry about the Mythics for now. If you haven’t already, get yourself into a good guild. A good guild will give you resources for chests. The more resources you have the more chests you can open which leads to a better chance of getting Mythics.
The other thing is Mythics are hard to get but there are plenty of Legendaries out there that perform just as good and even better in a lot of cases better than Mythics. Dragon’s Soul and Queen Ishbaala are two just to name a couple. Hell, Dragon’s Soul is the MVP on my pvp team. Not downplaying Mythics at all but there are Mythics out there that have no business being Mythics. Getting an awesome legendary like Dragon’s Souls is better than getting Gargantaur all day long.
Make sure you go for the daily diamonds. It’s not a lot every day but on Sundays they give a little more and they do add up. The Sunday diamond deals they offer will make the diamond accumulation faster if you choose to spend gems on them (again, the weekly guild rewards will help with this).
Overall I’d say don’t get caught up into the whole Mythic thing. Seems like you just started so I’d say just enjoy the game and build up your troops. Get some Ultra Rares and Epics and then concentrate on some on the Legendaries. That’s the majority of the fun. The Mythics are more of a bonus than the actual game.