I stopped with 200M. It is all a thing about factors. x2, x4 and so on. Should your income stay at a lvl that says: 6 month for Lin, you will need a x2 of your current speed to have 3 month. Should you need more than 3 month to gain said increase, it would be better to wait it out. If you need 30 days to double your speed, you will need 60 days after that to double it again. And 120 days after that and 240 days after that.
I did this with Anya, smaller amount of money... And when Lin came out, I had leveled my money buildings already and prefarmed some cash, so my time to wait was not 6 month but more like 1 month.
Leveling your income buildings gives you about a x2. And freeing up energy from skills maybe a x3 or x4 in total with leveling the buildings. Depends on your energy and what you have now allocated.
You can buy booster packs for diamonds now and get a free card every week.
Next thing I would expect is some kind of ladder overhaul where you loose points if you loose on the defence.