OK. Lets assume you drilled some wells and found some oil - lets say a couple of those wells are good, at 1000 barrels per day (if you're a Soviet and therefore metric, remember its 7 barrels to a ton). You probably have a rough idea of what formation you found that oil in. You don't know if they are small isolated pockets, or one connected field.
Therefore, more test wells and more mapping. You want to push out the limits of the field. This will get even worse if it's oil that "shouldnt" be there (like, well, Daqing. We still arent 100% sure of the age of that oil and therefore it's source rocks). This may well need more drilling rigs moved in and more crews trained.
You also are going to need a pipeline or railroad from that field to your refinery system. This may well involve building new port infrastructure as well. You're going to want decent quality roads as well.
Note you dont want to do this if you got lucky on three wells, and found three small enthusiastic 2000 bopd wells that ran for a month before starting to water out - so you are probably going to wait till you've done a lot of test wells and mapped the field correctly.
Once this is done, you may well need to 'tune' your refinery for the sort of oil you've now got - if you had an oil refinery that was designed to deal with heavy, waxy crude, then it's not going to like light but slightly sulfer tainted crude, and will need to be rebuilt.
You're also going to need to build water-handling facilities at your field, and probably housing for your new workforce, resupply and maintainence for your drilling and pipeline crews and so on.
So. Yeah. All this takes time.
From the demand side,
https://defense.info/re-thinking-strategy/2018/10/oil-and-war/ is pretty good.