For how long can you invest into bitcoin? You will get tired of investing into bitcoin and I will keep rising after that also. Bitcoin is now non ending process. The technology will keep advancing itself and mining efficiency will increase. Do not forget that not all of the available bitcoins are in circulation currently. Once we pass the time they might be released according to needs and this will increase the price of bitcoin ultimately. There will be more investors, more people getting aware of bitcoin and so on. The process will never end and with up and downs it will rise in price.
Any phenomenon can not be eternal and bitcoin also will someday die.
But I do not think that this can happen in the next 50-100 years. Now bitcoin actively develops and is accepted by many countries of the world. It's too early to talk about his death.