IR2110 max duty cycle
The duty cycle may not be 100% because the driver, like most of its type, uses a charge pump (bootstrap capacitor) to supply the high-side gate voltage (which has to be at least \$V_\mathrm{GS(th)}\$ above the voltage being controlled for an enhancement mode N-channel MOSFET). If left on indefinitely, the capacitor discharges through parasitic current paths, the gate voltage drops, and the high-side MOSFET enters the linear region and burns out due to excessive power dissipation. If you're unlucky, wild oscillation may occur in the process, potentially destroying the rest of the circuit. (Remember that MOSFET capacitances change strongly with \$V_\mathrm{DS}\$, so you can still get very severe oscillations long before you would expect the degree of discharge to be problematic based on a simple calculation.)

So, the duty cycle in itself is not the important factor, but rather the on-time. You can calculate the maximum safe on-time from the gate leakage current, the required gate charge for proper operation, and the capacitance of the bootstrap capacitor. In practice, you may need to measure \$V_\mathrm{GS}\$ using an oscilloscope (obviously, exercise due care and use high-voltage probes if you are working at 500V) and experiment to determine the parameters, since I don't expect that e.g. gate leakage vs. temperature is going to be specified in the MOSFET datasheet. Some of this leakage will occur through the driver itself, which is similarly unlikely to be specified in great detail.