I'm a new SC2 player trying to learn Terran. Right now, I'm practicing pretty basic builds like a standard OC opening, 1Rax FE, 1/1/1 with the Cloaked Banshee harass, etc. I've exercised most Terran units in the builds I've practiced so far with the very notable exceptions of Ghosts and Battlecruisers. I'm less interested in the Battlecruisers at the moment and am particularly interested in when Ghosts are effective and what I should be looking for to know when it is an appropriate time to transition into building a Ghost Academy and producing some of those units.
Since the recent Snipe nerf, the only matchup where you get ghosts is TvP. A good timing for getting the ghost academy and starting ghost production is around when you take your 5th and 6th gas. Before that you won't really have enough gas for upgrades, ghosts and constant medivacs/vikings and the Protoss won't have High Templars and Archons out anyways.
However, no matter the game state, as soon as you scout either High Templars, Archons or a significant number of immortals, you need to get Ghosts ASAP.