If your bonus is 24 (+20 and +4 for 2 spies) you get about 0.0946 chance to steal, so can expect 1 tech every 10.5 turns on average. If actual results are way off, then it can a bug. E.g. in your case the chance of not stealing for 30 turns is about 0.05, at this point I would start suspecting a bug.
I may have exaggerated, but it was at least 20 turns, can't remember exactly now.
I will make a new game with the same race picks and attempt to spy on the first AI I encounter. I can make a few tests like that, but for now bed time is long overdue for me.
I'll check it out tomorrow.
Played three games yesterday in 1.50 classic, to get a feel for the effects of different spying boni in-game.
Setup was small galaxy with 8 players, for early contact, on Impossible difficulty.
My race was Unification, Production +1, Large HW and Repulsive.
With: Game 1. Spy bonus -10; Game 2. Spy Bonus +20, Game 3. Spy Bonus +20 and Telepathic, for total +30 spy. (for game 3 i set the maximum_positive_picks to 16 to be able to get Telepathic)
- generated a map on which i was in contact with minimum 2 opponents.
- no expansion, playing on homeworld only, did not scrap colony ship but kept in orbit around HW
- from t0 all i did was build spies and put them on the offensive, for the entire game which lasted until t100
- if i got leaders, i did not hire them.
- no interaction with AI (was repulsive anyway)
-10 spy: During the entire game i only stole 3 techs. At end of the game I only had 3 spies because the rate of attrition was so high. I lost this game at turn 95 because i was at war with 3 AI (I wonder why...) and they destroyed my HW.
+20 spy: Stole 9 techs during this game, and at turn 100 I had 17 spies on the offensive spread over 3 opponents. Diplomatic stance with AI was poor but there were no wars!
+30 spy: Stole 27 techs, which is basically everything they had and i am sure I could have stolen more if they had had more techs (many turns, the AI had nothing for me to steal). I had 21 spies on the offensive and needed to set a tax rate to maintain them. I was at war with 1 AI, but alive and well at turn 100.