Is there any way to have an inserted plug-in only operate for a few measures? Like if I wanted to drag a plug-in from the Synth Rack plug-in menu to my track, but not have it in effect for the entire song, but terminate after a few measures?

OK the simple way is to cut out the part you want the synth to work with and past it into a track outing to the synth. 
 
 

Grundberg
Is there any way to have an inserted plug-in only operate for a few measures? Like if I wanted to drag a plug-in from the Synth Rack plug-in menu to my track, but not have it in effect for the entire song, but terminate after a few measures?

Several ways to do this

You can split the clip into a separate clip that is a long as the measures you wish the plugin to effect and insert a clip effect on just that clip

You may be able to automate the mix between the dry and the wet level so that the mix is 50/50 (or what ever ratio you need) for the duration of the measures.

You can create a stereo bus with the desired plug in on that bus and use automation to change the send level for the for the duration of the measures.

Others may have more elegant solutions...

promidi

Grundberg
Is there any way to have an inserted plug-in only operate for a few measures? Like if I wanted to drag a plug-in from the Synth Rack plug-in menu to my track, but not have it in effect for the entire song, but terminate after a few measures?

Several ways to do this

You can split the clip into a separate clip that is a long as the measures you wish the plugin to effect and insert a clip effect on just that clip

You may be able to automate the mix between the dry and the wet level so that the mix is 50/50 (or what ever ratio you need) for the duration of the measures.

You can create a stereo bus with the desired plug in on that bus and use automation to change the send level for the for the duration of the measures.

Others may have more elegant solutions...

The OPs talking about a synth not an fx.

1. Just create your part like any other synth track and only record for the duration you require at the appropriate location in the track.
2. Or do as 1 but bounce to audio and create a short clip that you can place correctly on the timeline.

mudgel

The OPs talking about a synth not an fx.

Some synths can be configured as effects