Please look at Help > About Audacity… and give us the actual Audacity version number (all three numbers).
At its simplest, click in the blue waves where you want to insert silence, Generate > Silence… and choose the length of silence to insert.
The only way I know of to automatically make different silence lengths of uniform length is to reduce the silence using Truncate Silence. However having made uniform short silences, you could extend them uniformly using the experimental Extend Silences plugin.
Note that silences inserted by either method are absolute silence. If it sounds odd to have “room noise” (the silence in your recording) followed by absolute silence, copy some “room noise”, click where you want to paste then Edit > Paste the room noise. The Punch Copy/Paste plugin may make this much easier, once you have the silences of uniform length.
Gale