We use serial4 for id column in the proxy_history table. That data type has 2147483647 (2^31-1) max value. When this values will been reached, zabbix proxy will stop with error "ERROR: integer out of range".

For example:
zabbix=# select currval('proxy_history_id_seq'::regclass);
currval
------------
2147521882
(1 row)

zabbix=# select (2147483647-2147521882);
?column?
----------
-38235
(1 row)

zabbix=# insert into proxy_history (itemid,clock,value) values (691037,1341975437,'13.000000') RETURNING id;
ERROR: integer out of range
zabbix=# insert into proxy_history (itemid,clock,value) values (60438,1341975438,'285.000000') RETURNING id;
ERROR: integer out of range