Maximum/ ideal length of ethernet cable?

There is no ideal length of a cat 5 cable but the specification states that it should not be more than 100m (328 feet).

More info on the wiki page

The specification of 328 feet has to do entirely with collision detection in a CSMA/CD (Carrier Sense Multi Access / Collision Detection network. Basically, the length is limited by the fact that the shortest possible frame size (64 bytes) can be sent out on the wire and if a collision occurs, the sending node will still be sending that frame when it hears the collision (due to a jam signal or higher than normal amplitude). If a collision occurs during the first 64 bytes of a frame, this is a normal collision. If something is out of spec such as the cabling is too long and the collision occurs after the first 64 bytes, this is a late collision and will not be retransmitted until an upper layer of the OSI model detects that the packet did not make it to its destination. You can run longer cabling and the network will still function, but there will be issues.

The ideal length is exactly as long as you need (up to 100 meters) and no longer. Every extra 11.9 inches adds another nanosecond of extra latency.

What is the maximum and/or ideal length of an ethernet cable? Is there a distance that data cannot be transferred over an ethernet cable, say over X number of feet?

Your question is really too broad. Different ethernet standards use different media with different lengths.

Some ethernet standards and the cable length limits:

10BASE-5       500 meters
10BASE-2       185 meters
10BROAD-36    3600 meters
10BASE-T       100 meters
10BASE-FL     2000 meters
10BASE-FB     2000 meters
10BASE-FP      500 meters    
100BASE-TX     100 meters
100BASE-T4     100 meters
100BASE-FX    2000 meters
100BASE-SX     300 meters
100BASE-LX10 10000 meters
1000BASE-T     100 meters
1000BASE‑CX      25 meters
1000BASE‑KX       1 meter
1000BASE‑SX     220, 275, or 550 meters
1000BASE‑LX     550 or 5000 meters
1000BASE‑LX10 10000 meters
1000BASE‑EX   40000 meters
1000BASE‑ZX   70000 meters
1000BASE‑BX10 10000 meters
1000BASE-T1      15 meters
1000BASE‑TX     100 meters
1000BASE-RHx     15 to 50 meters
10GBASE-T       100 meters
10GBASE-SR      300 or 400 meters
10GBASE-LR    10000 meters
10GBASE-ER    40000 meters
10GBASE-ZR    80000 meters
10GBASE-LX4     300 or 10000 meters
10GBASE-LRM     220 meters
10GBASE-CX4      15 meters
10GBASE-KX4       1 meter
10GBASE-KR        1 meter
10GBASE-PR    20000 meters

There are other ethernet standards with other cable length limits. Some cabling has other requirements, too. For example, UTP cabling has a limit of 100 meters, assuming 90 meters of solid-core, horizontal cable (better performance, but more fragile) and a maximum 10 meters of stranded patch cable (worse performance, but less fragile) split between both ends.

There are also different grades of fiber cabling, and some ethernet standards can be used at different distances, depending on the fiber grade, while others require a specific grade of fiber.

Ideal: As short as possible, as isolated as possible.

Maximum: 100 meters without repeaters.

To be in spec, a CAT 5 UTP cable should not exceed 100 meters. Cable can be connected with repeaters and you can get another 100 meters and so on. However, if you network is too large, then the TCP/IP packet will take so long to go from end to end computers will reach the timeout before they get a reply back. At that point other devices will have to be used to retransmit the packets, like switches/routers... I'm not sure about that distance/time before packets are considered lost.