I'm going to assume the RTC is either a separate chip with its own crystal, or a module integrated with your microcontroller that again has a separate time source (such as as a 32 kHz crystal) than the main clock. And the time source for the RTC is more accurate than the one for the microcontroller.
To determine how often you need to read the RTC, you need to figure out what the maximum error your main clock can have. For example, if the main crystal is spec'ed at 20 ppm, that is the same as 0.002%. So a clock just based on the main clock source could drift 0.00002 * 3600 * 24 = 1.728 seconds a day.
So if you read the RTC only twice a day, and in between incremented the time once a second using a timer interrupt, you would never be off more than a second -- never be off more than a second compared to the RTC that is.
If, as I assumed earlier, your RTC is either a separate chip with its own crystal, or a module integrated with your microcontroller, that doesn't mean it is correct. An RTC can have an error too. For example, if it is using a 32 kHz crystal with a tolerance of 5 ppm (which are just slightly more expensive than 10 ppm ones), it could be off by 0.43 seconds per day -- or 13 seconds per month.
To get around this, you will need to tune the RTC, where you write a correction factor back to a register. Doing so will allow you to get the error practically to zero. But of course you'll have to have a third external clock source to use as a reference when doing the tuning. An extremely accurate reference in the US is the 60 Hz AC line, which is guaranteed to be exactly 60*60*60*24 (5,184,000) cycles in a 24 hour period between successive midnights. For this to be useful, you must time for the entire 24 hours, as the 60 Hz can drift some between midnights.
Another excellent time reference would be using GPS (10 ns accuracy), if one already had GPS hardware in their project.
If instead your RTC times comes from an external source, like the cellular network time (AT+CCLK? call), or a network time server using NTP, then you can use the RTC value as is since there will be nothing to "tune".