For me, it was weekends. I'd found early on that my wife would say, "Just go ahead and get it," even with stuff I was on the fence about, when she was with me at the hobby shop in Tacoma. But if I walked in the house after going and she didn't know ahead of time, I wouldn't get a warm welcome. As long as she knew ahead of time, I was able to get away with a lot. In fact, we were at the gift shop for the White Pass & Yukon RR last month and I'd warned her I'd probably buy a but o' swag there. When I brought stuff up to be rung up, she saw there wasn't all that much and said, "That's all you're going to get?" I bet the nice gal at the cash register didn't hear that said often. She'd expected me to buy a lot of stuff, including a DVD set I'd assumed they had for sale there (which they didn't, I bought it online a couple of weeks later though).
But discipline keeps you from going broke. I model a very specific prototype RR that never interchanged with any other and didn't have a lot of rolling stock by the timeframe I'm modeling. Anything excess really wouldn't make sense, even though I bought an extra locomotive that was off the RR a year before my layout's timeframe (it is, like most layouts, an alternate reality representation).
Having collected various non-RR-related things for my adult life, I know how to set tight goals and stick with them.
I've found through that, that it's always best to have 2 'way cool' things than 10 'so-so' things. You can get to a point where the stuff owns you, and to me, that's no fun at all.
I rarely find wisdom in song lyrics but I live by this one:
It's not having what you want
It's wanting what you've got
-"Soak Up The Sun" by Cheryl Crow