diff --git a/R/logging.R b/R/logging.R
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+#' Logging utilities for RKorAPClient
+#'
+#' This module provides centralized logging functions used throughout the package
+#' for progress reporting and ETA calculations.
+
+#' Log informational messages with optional coloring
+#'
+#' @param v logical flag indicating whether to output the message
+#' @param ... message components to concatenate and display
+#' @keywords internal
+log_info <- function(v, ...) {
+  green <- "\033[32m"
+  reset <- "\033[0m"
+  cat(ifelse(v, paste0(green, ..., reset), ""))
+}
+
+#' Format duration in seconds to human-readable format
+#'
+#' Converts a duration in seconds to a formatted string with days, hours, minutes, and seconds.
+#' Used for ETA calculations and progress reporting.
+#'
+#' @param seconds numeric duration in seconds
+#' @return character string with formatted duration
+#' @keywords internal
+#' @examples
+#' \dontrun{
+#' format_duration(3661) # "01h 01m 01s"
+#' format_duration(86461) # "1d 00h 01m 01s"
+#' }
+format_duration <- function(seconds) {
+  if (is.na(seconds) || !is.finite(seconds) || seconds < 0) {
+    return("00s")
+  }
+
+  days <- floor(seconds / (24 * 3600))
+  seconds <- seconds %% (24 * 3600)
+  hours <- floor(seconds / 3600)
+  seconds <- seconds %% 3600
+  minutes <- floor(seconds / 60)
+  seconds <- floor(seconds %% 60)
+
+  paste0(
+    if (days > 0) paste0(days, "d ") else "",
+    if (hours > 0 || days > 0) paste0(sprintf("%02d", hours), "h ") else "",
+    if (minutes > 0 || hours > 0 || days > 0) paste0(sprintf("%02d", minutes), "m ") else "",
+    paste0(sprintf("%02d", seconds), "s")
+  )
+}
+
+#' Calculate and format ETA for batch operations
+#'
+#' Helper function to calculate estimated time of arrival based on elapsed time
+#' and progress through a batch operation.
+#'
+#' @param current_item current item number (1-based)
+#' @param total_items total number of items to process
+#' @param start_time POSIXct start time of the operation
+#' @return character string with formatted ETA and completion time or empty string if not calculable
+#' @keywords internal
+calculate_eta <- function(current_item, total_items, start_time) {
+  if (current_item <= 1 || total_items <= 1) {
+    return("")
+  }
+
+  elapsed_time <- as.numeric(difftime(Sys.time(), start_time, units = "secs"))
+  if (elapsed_time <= 0) {
+    return("")
+  }
+
+  avg_time_per_item <- elapsed_time / (current_item - 1)
+  remaining_items <- total_items - current_item + 1
+  eta_seconds <- avg_time_per_item * remaining_items
+  estimated_completion_time <- Sys.time() + eta_seconds
+  completion_time_str <- format(estimated_completion_time, "%Y-%m-%d %H:%M:%S")
+
+  paste0(". ETA: ", format_duration(eta_seconds), " (", completion_time_str, ")")
+}