In my opinion, investing in cryptocurrencies is very important because it is clear that crypto will take over the world. The use of cryptography in the world of finance allows financial systems to operate at a global scale whereas the current financial system is controlled by a handful of powerful banks and countries. Crypto will open up the world of finance to the world. As crypto advocates say, it will "bank the unbanked."
Nevertheless, it is important to be careful out there in the crypto world. My advice is to invest no more than 10% of your net worth in crypto. Having nothing in crypto is dangerous because crypto is likely to take over the global economy in the future to a significant degree. If you have nothing investing, you'll miss out on a once-in-a-lifetime opportunity to get super rich.
Nevertheless, crypto is the wild west of finance. There is great opportunity but great risk as well, so investing no more than 10% of your net worth is my recommendation.
When investing in crypto, make sure you spread your money across the top cryptos. A favorite website for crypto enthusiasts is coinmarketcap.com, which lists the top cryptos by market capitalization. You generally want to invest in cryptos that already have a reasonable high market cap as this is an indication by the free market that the crypto is high quality.
Another issue is storage of crypto. I don't think holding crypto in an exchange is that dangerous so long as the exchange is trustworth, but never put too much in any one exchange. For example, Coinbase's "vault" feature looks very secure, and Coinbase is a reputable firm. Nevertheless, MtGox was considered a reputable business before it was hacked. The best way to store crypto is to hardware wallets like Trezor and Ledger Nano S or even paper wallets.
Regardless of what you invest in, remember that what is most important is how much you save. You may be investing in the best investments but if your savings rate is only 5% of your pay then you're not going to be investing much. Increase your savings rate. This is achieved by living a minimalist lifestyle. Focus on a minimalist and frugal lifestyle that gets rid of expensive leisure activities. Focus instead on free and meaningful activities e.g. reading free public domain ebooks e.g. read the entire works of Arthur Schopenhaur. An easy way to get free ebook is to put "EPUB" at the end of a google search. EPUB has nothing to do with beer! EPUB is the file format for ebooks. For example, google search "Arhur Schopenhaur EPUB," download the EPUB ebooks onto your computer, sign up for a Google account and then upload those EPUBs to the Google Play Books website. Then install Google Play Books on your tablet and/or smartphone, sign in, and and your ebooks are available for you to read anywhere.
I don't think there should be a fixed % -- however, people invest what they can but with due diligence. Never invest more than what you're comfortable losing.
If the thought of losing the money you're investing makes you feel seriously anxious, it is either too much or with the wrong company. This is how I gauge it -- I don't invest more than what I would consider an "annoying inconvenience" if I lost. :)