You just need to have a good money management skills to be able to invest in Bitcoin because even though you have a source of income, if you cannot manage your finances properly, you may be caught in a very difficult situation in the future which might result in you tempering with your holdings, that's why a good money management skills is required in Bitcoin investment, so that from the little you are earning either weekly or monthly you can still keep your emergency funds out of it, and possibly a reserve funds, because in other to be able to hold successfully and on the longer term, all these needs to be in place, so that you wouldn't have to temper with your holdings if a serious emergency situation arise in the future.
If you are looking to invest for long term like 5 years or more then invest what you can afford easily weekly or monthly. If you unease yourself by investing large portion of your earning into bitcoin then you will stop investing after few weeks or months. It's very important that you keep an adequate amount for your daily living and also for emergency funds that you can consume in case of emergency requirements. After that we can invest what is left behind. By following that strategy you can continue to invest in bitcoin for longer duration. Don't just rush to invest everything you have into Bitcoin but plan on how to invest.