There are two answers to this question:
1) Are you trading interest rates futures through the CME?
Electronic trading through the CME for interest rate futures is open 23 hours per day, Sunday night thru Friday evening.
“Bond Market Hours” for the CME are 6:00 PM – 5:00 PM EST.
2) Are you trading bonds through an online broker like Fidelity or Schwab?
While you can access your account online 24 hours per day, your are not likely to be able to trade anytime your want. You account is likely to have live, executable prices for a small portion of the day.
From a practical perspective, the bond market hours are from around 8:00 AM – 5:00 PM EST.