I think the more important question to address is what your overall savings rate should be. Where is goes is a secondary issue after you establish what your total savings rate is going to be.
So, without knowing more about your situation such as income, liabilities, assets already saved, whether you have an emergency fund established, goals, etc., it's impossible to say for sure. However, as an absolute bare minimum, I generally suggest 10% of one's gross income as an overall savings rate (whether it all goes into your 401k or goes into the 401k and other accounts is a different issue, touched on a bit toward the end). But, I usually use 10% ONLY for those just starting out in the first job/career who may be at lower salaries and still have to keep a roof over their head, pay student loans, etc. or for people who have never saved before as a way to start building the habit.
In a perfect world, by age 30, you would be saving a minimum of 15 of gross income, and ideally, closer to 20%. If you cannot start there right now (say you can only start at 10%), I would make it a goal to be saving 20% of your gross income by age 35. One way to do that is each time you get a raise (either from the company you start with, or by changing jobs), you increase your savings rate. So for example, if you start at 10% at age 30, and get a 3% raise after your first year, increase your savings rate to 12%. Then repeat that process until you are at least to 20% of your gross income (if you repeat it every year for 5 years in this example, you would be at 20%).
Given your age, while I do believe that Social Security will be around for a very long time in some form, I often tell people to plan as if it won't be around at all. If you can save/invest enough to ensure a solid retirement on those funds alone, any Social Security benefits you do receive will be an added bonus and probably mean that your savings/investments will last that much longer.
The earlier you can start saving, the longer you have for that money to grow and compound. So, if you are just starting and can do a 15-20% savings rate (with a goal to get to at least 20% no later than age 35) and keep that throughout the rest of your career, while there are no guarantees in life when it comes to investing and retirement, it should put you on strong footing for a comfortable retirement.
As for what you should be putting those savings into, that is really a different discussion, but I would suggest a few options to consider (you can work on all at the same time): (1) establish an emergency fund covering at least 3-6 months of living expenses (perhaps more if, for example, you are the sole earner in the family with a wife and 3 kids; but in any case, no less than 3 months worth of expenses) in a plain old savings account - once you have that established, you can take the money that was going into that and move it to other options, (2) defer at least enough into the 401k to get the full company match, if they provide one (and don't use the match in the savings rate calculations - treat it like Social Security where the match is simply gravy on top of the amount you are personally saving), (3) if your income does not prohibit you, utilize a Roth IRA (ideally in conjunction with a 401k) so that build up an after-tax pool of money that will be tax-free when you withdraw it in retirement (while the 401k will presumably be pre-tax so you get a deduction now but have to pay taxes on withdrawals in retirement), (4) if you have fully utilized #2 and #3 and still need/want to save more, a regular, after-tax account will work.
As I said, it's impossible to say for sure without knowing more about your overall situation, but I hope this helps some, and best of luck.
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