As a US citizen, you can stay outside the US for as long as you like without any fear of losing your US citizenship simply for not being in the US.
As for losing financial benefits, some programs require the recipient to be living in the US. Some programs have no requirement that the recipient to be living in the US. The benefits that usually require residency usually are needs-based (such as Supplemental Security Income). The benefits that usually do not require residency are usually entitlements (such as Social Security). You should check with the office that administers the benefit to find out the requirements.
If your benefits are administered by the Social Security Administration, you likely can find your answers from the SSA's website: http://www.ssa.gov/ .
You can be outside the USA for as long as you wish now that you are a US citizen. You can also petition for permanent residency (the green card) for siblings and married children, in addition to unmarried children and your spouse.