I started to pay close attention to BTC 3 weeks ago and its price was about $7000. And up to now its price was $12500, reaching it's peak. Is it appropriate to invest in bitcoins from now on？ 
Actually it depends upon you that for which purpose you are looking to invest in bitcoin .If you want buy bitcoin to hold for long term then definately you Should buy now and hold for atleast 1-2 year to make big profit .
Second thing if you are looking to buy and use that amount of bitcoin for making trading then I would like to suggest you to wait until the price of bitcoin get stable , otherwise you will face problem of unstabilities in this field and can face big loss and in that case you will needed long term wait for profit in your investment.