I just assume that it will depend on the kind of wallet that you use, its very rare that wallet developers can just decide to shutdown their operations and go away with people's investments. However, I guess you just have to ensure that the wallet is secure enough and you have control of your private keys. I personally think that there is too much risk if you will be storing your bitcoin on an online wallet for long term. If you are willing to hold on to your bitcoin for long term, rather buy yourself a hardware wallet or rather use a desktop wallet instead.