The Bluetooth Specification specifies that the advertising interval should be between 20 ms to 10.24 sec. If you want to be compliant with the BLE Specification, I recommend that you don't try to exceed these limits.
That said, you can stop the advertising with the function sd_ble_gap_adv_stop(), and using the advertising module you can start with the function ble_advertising_start(), or directly with sd_ble_gap_adv_start(). You can then set-up a app_timer to control when you want to advertise.
