It's hard to say, because we don't know how high your ammonia or nitrite might be.
I'd be tempted to do one large one to start with; leave just enough water for the fish to swim upright (don't forget to switch your heater/filter off first!) before refiling with warmed, dechlorinated water. That'll get rid of anything that's built up since you last did one.
Then do 50 or 60% every day until you can afford some test kits. If you get the shop to test the water, get them to write down the actual numbers for you; sometimes they'll just say, "it's fine", or "a bit high", which really doesn't help!
Best of luck; keep us posted on how things are going
I'd be tempted to do one large one to start with; leave just enough water for the fish to swim upright (don't forget to switch your heater/filter off first!) before refiling with warmed, dechlorinated water. That'll get rid of anything that's built up since you last did one.
Then do 50 or 60% every day until you can afford some test kits. If you get the shop to test the water, get them to write down the actual numbers for you; sometimes they'll just say, "it's fine", or "a bit high", which really doesn't help!
Best of luck; keep us posted on how things are going
