League of legends wikia fixes the number of souls required to match other champions armour:
At level 18, it takes 67 souls (roughly four per level) to surpass Orianna's 62 armor, the lowest innate champion armor. It takes 140 souls (roughly eight per level) to surpass Taric's Shatter-enhanced 116.6 armor, the highest armor possible without the use of items, runes, or masteries. source
I find this "Number of souls per level" to be a bad metric, since level progress is not linear. We can read in the same page that:
On Summoner's Rift, the drop rate for souls from small minions is 33%. However, the probability adjusts dynamically. If you are above or below your expected quota of soul drops from small minions, the probability lowers or rises by a small amount until you are back at your quota of drops. It is worth stressing that this only takes into account souls dropped, and not souls collected.
This means each wave will drop 2 souls plus one each three waves.
Considering this, I computed this spreadsheet.
Theoretically you can reach the lowest armour point at 15m30s if you can collect all souls, and the highest at 31m. This theory is impaired by the zoning of the enemy. They will make some souls not available to pick up, force you out of lane losing souls. In return, your get bonus by killing, or being under the quota.
The worst thing that can happen, regarding soul collection, is being out of lane (death, roaming, back to shop). Thus this time has to be minimised. In consequence, you must be careful when picking souls if the enemy bot lane can harass you. Unless you can take the shots and get your health back after it.
If we try to read the statistics the other way around, we can have a view of what we can afford to lose.
The theoretical maximum at level 18 is 189 souls. Subtracted from the 140 of the highest champion armour at level 18, remains 49 souls. This is the number of souls you can afford to lose "without penalty" when compared to Taric. Losing more than 122 souls would put you below any champion armour rate at level 18.
49 souls are roughly 21 waves out of 81, which means you can afford to lose a forth of the soul drop. 122 souls are 52 waves, approximately two third.
Conclusion, if you lose more than 2/3rd of the souls, you are below the lowest champion armour rate. If you lose only 1/4th of the souls, you match the best champion armour rate.
Bonus : how to collect souls efficiently
The enemy ADC will always try to shoot you if he can: try to look for his last hit timing. This will force him to chose between harass and money. Usually They'll get the last hit (because money is more valuable and because that was the plan in the beginning)
The enemy support will also try to get hit on you. Avoiding his zoning is a bit more difficult. Try to get a good position, take advantage of the zone he is not covering to come and pick the souls from the farthest point away from him.
Their are many souls but they are too far away? If the situation is calm, throw a lantern, it can collect souls for you. Avoid overusing this technique as your carry might need the lantern for shielding or move away.
You need to go back home for health, but you ADC is still around?? Drop a lantern before hitting
B, this will collect any souls that drops before you are sent back to base.