For my huge battles reinforcements are usually a must - it’s too much to have everything on the board at once - Normandy, Kursk, Battle of the Bulge, and the upcoming Pacific Island battle are all using waves of reinforcements. You can do it without reinforcements of course, but if you do the turns can take forever. In smaller games unless its part of a scenario (hold out until the Shermans arrive, etc) I don’t use them.
How much the waves are depends on a few things - do we want it to be a tug-of-war where sides alternate between having the ‘strongest’ forces, or just one extra wave, or multiple ones.
For example our Normandy game had the Allied landing forces 3:1 to the Wehrmacht forces defending the beach, and their Paratroopers/Resistance forces were around 1:1. However the landing forces came in waves every other turn until they were all used up, and the Germans got a huge reinforcement but it was split up into 3 groups and each had to roll a certain number before it could come on (D6 + Turn Number > 8 I think it was).
In Kursk both sides had reinforcements on set turns, and only a fraction of the German force began on the map; every 2 turns another wave came on (Recon, Panzers, then support weapons). The Russians also had reinforcements come on when the Germans got their largest wave.
Battle of the Bulge was pretty simple; both sides recieved reinforcements turn 5.
In our Pacific Battle tonight the Japanese get no reinforcements, and the US land in higgins/amtracks/sherman DDs - they get 6 reserve higgins in case the others are destroyed. The US has a set amount of forces; if they run out then too bad. After a Higgins drop off troops the US side can remove it and move on a new loaded one next round, so the arrival of the waves is determined by the players to a degree (if they want to take a direct route, or go around the side of the island).