Hello Believer,
I can see now that I missed an ambiguity in your example:
1. If you've had a cold for a month, you should go and see a doctor.
— in the context of "general advice", this is a variant on the type 1 conditional. The present perfect replaces the simple present; "should" replaces "will".
2. "I've had a cold for a month." "If you've had a cold for a month, you should go and see a doctor."
— as in my previous post, here, "if" is very close to "since"; the speaker accepts the fact expressed in the if-clause.
Your later sentence isn't quite idiomatic, as it stands; but you could rephrase it as:
3. If you have won £200,000 in the last month, you should go out and buy a new watch.
Again, this could be regarded as general advice, and therefore a variant on the type 1 conditional, as in #1 above; or it could be a similar case to #2, where "if" is very close to "since":
4. "I've won £200,000 at the races, in the last month." "If you've won £200,000 in the last month, you should go out and buy a new watch."
The difference between #1/#3 and #2/#4 is one of focus. In #1 and #3, the speaker expresses an implication; in #2 and #4, the speaker expresses an inference.
Would it help if I gave you some more examples?
MrP