In christianism, grace can be different things : an experience of the divine, an ability in a certain domain, or even the release of an addiction to cigarettes for exemple. All those things have one thing in common : they are given by God, and he only can give them. They never appear because of our efforts, they are just given (and we never deserve them). However, our efforts are necessary as a premilinary stage. For exemple, the three "vertues theologales" (sorry but I don't know the english name) which are faith, hope and charity are considered as graces, which can be given only if we have already developped the four cardinal virtues , wich are carefulness, moderation, strenght and justice.
In general, grace are only given if some efforts in the right way have already been done (generally contemplation). These efforts are necessary, but non sufficient, and that's why it is said that they are a gift (we don't deserve them).
In buddhism, we can have such things as realisations, or sudden things that arise in us. In fact, we call them sudden things, but we know that they only appear because of our work. So, for exemple, we meditate, we meditate, and we meditate again, and suddenly, something happen. There was no way to say that it would happen at this moment, but we know it was caused by our meditation. The Buddha said that it can be difficult to know at what point we are in our progression, and as it can create wonderings, we should just "do the work" without searching to know. If we're doing fine, the results will appear, sooner or later.
So, for both christianism and theravada, we cannot say when the "graces" will appear, or even if they will appear. But the difference is :
- in theravada, we think that these things are completely due to our practice (even if they seem to be sudden)
- for the christians, they are not caused by our practice, they are gift from God.
I'm not a specialist of christianism, so you will have to verify and adapt what I said about this...
Please don't hesitate to correct my english if you feel to