The best time to apply lime, if it is needed, is in the fall. While the lime does start to react, if moisture is present, right away it takes several months, maybe even a year or so, for the full affect to be seen.
Why do you think lime is needed?
What is your soils pH?

The "fast acting" limes I see in stores are complete ripoffs and vastly overpriced. They are the same as the cheap lime with maybe just a very slight increase in the finest grains to allow the largely fake claim.

Quicklime (CaO) and slacked or "hydrated" lime (Ca(OH)2) are available in some places and do act faster (and can easily irritate the skin and nose and lungs) but are probably not worth the effort.

Powdered limestone (with little mangesium, Mg) will dissolve and affect pH faster than the nutritionally more valuable powdered dolomite or dolomitic limestone (with significant Mg).

Many fields around here have excessive Mag due to years of blind apps of dolomitic limestone.

"Why do you think lime is needed?
What is your soils pH?"

I have clay soil which are mostly acidic from what I understand.

Should I go out and spread it now?

Clay soils are not necessarily any more acidic then sandy soils. Get a good reliable soil test before applying any lime so 1) you will know which, Calcitic or Dolomitic, lime you need, and 2) you know how much you need to add.
Simply applying some lime of some kind to your soil may wello be a waste of your money, time, and energy if you do not add enough of the right kind to correct the needs of your soil.
Plants need both Magnesium and Calcium to grow up strong and healthy but an imbalance in your soil can cause the plants to not be able to utilize either one in a way that promotes strong and healthy growth.

I would especially not add dolomite unless it definitely needed it.

Just a little something else to keep in mind. The "ag" lime is loose & sort of powdery. If you have a way to haul small amounts (1 ton) it is MUCH cheaper than the bagged stuff. We paid $20 for 1 ton. It is harder to spread & does blow if it is windy but it gets working faster and can last up to 3 yrs. in your soils.
But Kimm is right...you need to get a good soil test first to see exactly what you need. I got ours thru our local county extension office. They have a tool I could borrow and they send the samples to the UofGa. for testing...results in about a week...cost $6 per sample. They also have bags for the collection.
Not everyone would want to mess with the ag lime but it sure is a LOT cheaper than buying several bags.

20 a ton!!!!! Deal. :)

Yea the powder should act quiker, how long do pellets take?

My guess is that the pelleted lime would work nearly as fast...everything else being equal.

Some people just add lime to their compost piles and don't need to apply it any other way. Also, it seems like it would be broken down into a more usable form during the composting process.

Halfway through my compost process, while making my final aeration, I add lime, greensand, kelp meal and rock phosphate in thin layers. This way I dont have to haul all these minerals out into the garden and have them blow around in the wind.

Unlike the greensand and rock phosphate that must break down or at least partially dissolve before beneficial use by plants, with the lime it is mainly the actual process of breaking down that IS the main use, here being pH adjustment in soil. What you do is just fine for the secondary use of lime for adding calcium (and mangesium when using dolomite) but for holding or raising soil pH the reaction usually would be better in the soil itself.

The actual reaction and dissolution of lime essentially extracts acidity.

Many people around here waste a lot of money by applying lime to their lawns every spring. The Calcium Carbonate in soil gives up ione of its O ions to one of the free radical Hydrogen ions and that is what changes the soils pH. While some of the CaCo3 reacts fairly quickly most of the exchange takes several months so to make a change in your soils pH means you should be applying the proper amount several months before you want the pH to be were you want it to be.
You can also add a lot of CaCo3 and then find your plants not growing strong and healthy because there is a deficiency of Magnesium, something plants need to properly utilize Calcium. A good, reliable soil test is needed to know what the levels of CaCo3 and Mg are and if they are in balance. Too much of one will create as many problems as too little of either one.

Yea you are right. Compost alone top dressed will keep everything ok.