It appears to depend on which service you are requesting. Like Drakama, I have yet to reach a limit when requesting the official apis: api.guildwars2.com, render.guildwars2.com, tiles.guildwars2.com. However I have run into issues with the unofficial trading post api: tradingpost-live.ncplatform.net. After only a couple(25) requests, I start to get response code 500s from the server. I then have to wait about a minute before I start receiving response code 200s again. I have yet to find an acceptable rate to avoid these response code 500s.