There are so many variables associated with this question that there is no one correct answer.
*What sort of retirement income will you have aside from your 401k?
*Will your 401k be fully invested in stocks (or the same thing, mutual funds
) for the next 19 years?
*Will the stock market continue to go up forever?
*What rate of return
will your 401k provide you between now and then?
*What retirement income from your 401k are you targeting?
*Will you need your 401k funds for hardship withdrawal, house acqusition, or other major expenditures between now and then?
*Will you start a business or want to make other large investments between now and then?
*Will you need to care for aging parents or other seriously ill relatives?
*Will your earning potential continue to grow
(You can see there are many other questions that could be asked as well)
To properly assess how much of your savings should be "locked up" in a 401k would require an overall assessment of your career history, career plans, housing situation, family situation, and other factors.
You are wise to be considering this question at this time - but it would be even wiser to do so as a part of an overall financial plan for the future.
HOWEVER, if you decide to consult a financial advisor, do so ONLY for planning purposed. DON'T automatically buy whatever products they claim will help your retirement planning. Separate your planning sources from your investment sources, it is more likely you will get better independent advice that isn't based on you being seen only as a source of a fat commission check.
Specifically, if you want even LESS flexibility with your money, an annuity will eliminate even more of your options. The single gravest danger to people's financial flexibility is to accept an advisor's recommendation to buy an annuity from that advisor. Be aware!
Good luck with your retirement planning.