There is no standard investment in bitcoin it depends on the set percentage based on the term and reference of the company you want to invest your bitcoin. Some companies or ICO's offered a small amount of investment but with high profit. It is depend on the set terms and condition and make it sure your investment is secure.
I Second that opinion there no set standard... Its subjective based upon how much you are willing to invest; how long are you willing to hold; what are the profit margins you are looking at... while there are several ways to invest and stay invested i.e. Holding bitcoin longterm in paper wallet, trade bitcoins on exchanges for other crypto currencies and investing in ICO's of good promising project (you would need proper awareness or guidance if you decide to invest in an ICO)
Regardless of what amount you are investing you should at-least hold 30% of your investment long term... considering the fork drama happening again by the end of the year may be hold 50% of investment as bitcoins on core node or any wallet as long as you have the private keys access so that you can profit from free coins if it happens again..