Which TTL? The TTL that comes from the upstream DNS server for any returned DNS request? Or, the TTL that Pi-Hole provides with any domain it blocks?
Note that you won't save much time in DNS lookups with a long TTL. If you dig a random domain from Google or Cloudflare, the lookup time is typically a few tens of msecs and the returned TTL is reasonably long.
Examples:
dig pi-hole.net @1.1.1.1
...
pi-hole.net. 1742 IN A 206.189.252.21
;; Query time: 18 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
dig pi-hole.net @8.8.8.8
...
pi-hole.net. 760 IN A 206.189.252.21
;; Query time: 30 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
In the dnsmasq manual page, this note appears with the min TTL command:
"--min-cache-ttl=time
Extend short TTL values to the time given when caching them. Note that artificially extending TTL values is in general a bad idea, do not do it unless you have a good reason, and understand what you are doing. Dnsmasq limits the value of this option to one hour, unless recompiled."