Learn English – What does “it gets old pretty fast” mean in these sentences

We can't assign a percentage of certainty to it, but it isn't close to 100%. Perhaps 60-85% certain is close to what it means for most native speakers. The more emphatic the statement (stress and pitch and volume are involved, because this is primarily spoken English, but in written English it might be underlined or in bold font or in capital letters for emphasis), the more certain the speaker/writer is. I'm quite sure probably means about 90-100% certain, however.

"If it gets too much" is a phrase meaning "if the situation becomes too difficult to deal with". In your example, Pat is saying that if the herd sale becomes too stressful, she has plans with Helen to go somewhere else instead.

Similarly you have the expression "It's just too much" for when a situation has already reached the point of whatever "too much" is in that context. Usually this refers to a negative situation (as in your example, where Pat wants to have an escape in case the sale is too stressful), but that's not always the case. For example if a surprise party is thrown, and the recipient is very surprised and happy, they might then also say "It's just too much".