I've seen it clear within a couple of hours on its own, but I was using a two hour time window. Double check your time window. It can be up to a month.
LLDP and CDP info is sent to the dashboard every ten minutes.
N.B. I do primarily interact with CDP via the API, because I'm often needing it for all devices in a network including the MX. You have to explicitly declare a range in seconds of less than one month there.
You can do a quick test of what the API endpoint sees in Postman. You'll need to:
1. "List organizations that the user has privileges on",
2. Make an "organizationId" variable and populate it with the org id.
3. "List the networks in an organization.
4. Make a "networkId" variable and populate it.
5. Make a "serial" variable and put your device's serial number into it.
6. Then use Device/"List LLDP and CDP information for a device".