Trade conference spending should fit the overall strategy. The more important it is for generating sales, contacts or leads, the more you might reasonably spend. Never forget that what you spend on this part of your budget rules out spending it somewhere else.
Always look for your most reasonable spending opportunities, using your best judgment to compare what you get to what you spend.
Then make sure to manage those forecasts and budgets by reviewing them every month to compare what actually happened to what you thought would happen.
There is no general rule of thumb. It's a matter of making your spending match your strategy.