Short Answer: Should be under an hour, but most likely too many other dinos exist for more to spawn where you want. Alternatively, players or structures are too close to the spawn location.
Long Answer: I don't have a concrete answer for how long do dinos take to spawn, and am not sure the developers have released detailed information*. Personal experience and other sources suggests less than an hour. I can summarize some relevant factors.
Note, with Single-Player or Non-Dedicated Multiplayer Servers it is a known issue that Dinos don't frequently spawn in caves.
On all servers are Server Settings that would impact dino spawns.
DinoCountMultiplier - Impacts total number of dinos spawned on the server
DinoSpawnWeightMultipliers - Impacts spawn rates of specific dinos, and capable of limiting them to a percentage of the total dinos. I don't believe percentage limiting is on by default.
The following are defined within Game.ini and could affect a specific location.
ConfigAddNPCSpawnEntriesContainer - Adds a specific type of dino to spawn list for a location
ConfigSubtractNPCSpawnEntriesContainer - Removes a specific type of dino to spawn list for a location
ConfigOverrideNPCSpawnEntriesContainer - Basically combines the two previous settings for a location
From the above we will look at issues in terms of likelihood.
First, if the server has spawned the maximum number of dinos, it will not be able to spawn additional ones for a new location. The server may despawn existing dinos to allow new spawns, but this not guaranteed. This could easily happen with default settings. Note that the sarcos and fish in the cave are taking up slots that could be used by other dinos.
Second, if the server has changed
DinoSpawnWeightMultipliers and set a
SpawnLimitPercentage within it to say, 0.10 for all dinos, then no dino can take up more than 10% of the global population. This means that you could max out the number of Titanoboas in the swamp, and they would be unable to spawn in the cave. Alternatively, the server may have reduced the rate of the specific creatures you are looking for.
The least likely option is that the server has used
ConfigSubtractNPCSpawnEntriesContainer to prevent non-fish dinos from spawning in the cave. It could be done, but would have to be done intentionally by an administrator.
An alternative issue is that a player or structure is blocking the spawn location. (Credit to @Dpeif for mentioning this in the comments) From a developer comment on Steam forum:
Dinos have a radius around human made structures that they can not spawn in
I have not found a definitive answer on the range for spawn blocking, but there are many claims it is 75 meters, as this was the radius for buildings preventing resources from respawning.
A final possibility is that the server is having performance issues, due to having a large number of structures, dinos and players, relative to its hardware/performance capability. This is solely based on the developer response below.
*Relevant response from the developers Ark digest 25
Q: Also can you explain how the dinosaur spawning and persistence works?
A: Wild creature spawns/despawns can be randomized based on a number of
factors, including server performance.